Attached to a machine with tubes disappearing into her arm, Patricia LeBlack, dressed in a loose cotton top that keeps slipping off her narrow shoulder, might look frail but when she speaks there is nothing feeble about her. She shouts to the man in the bed opposite, on the renal ward of the Royal Free hospital in north London, who is hooked up to an identical machine, and she jokes with the staff, who call her "Mama". She has known many of them for years but the younger staff were not even born when LeBlack started her treatment here.

Her survival has been remarkable. This is her 39th year of continuous kidney dialysis and her doctors believe she is the longest-surviving dialysis patient in the world. When LeBlack was first diagnosed with kidney failure at the age of 27, her doctors gave her 10 years to live. This year she celebrated her 65th birthday. It is difficult to give an average survival time for dialysis patients because so much depends on age and health. Many are elderly when they start dialysis (their kidney failure is usually a complication of diabetes) and so don't live long, but an otherwise healthy person starting dialysis in their 20s can expect to live upwards of 20 years without a transplant. Dialysis patients may also be more susceptible to pneumonia and infections, and many find it hard to stick to the essential strict diet and minimum fluid intake, which can put pressure on the heart.

LeBlack came to London from Guyana on November 14 1963 to train as a nurse but when she arrived there were no places left on the course she had planned to take. Instead, with her step-sister, who already lived in the capital, she went round the factories looking for work. Eventually she got a job at a brewery in King's Cross. LeBlack became pregnant but didn't stay with the baby's father. It was while she was pregnant that her doctor discovered that both of her kidneys were not functioning properly. Renal failure can happen for a number of reasons - after an operation or an accident, as a side effect of medication or a complication of diabetes - but nobody knew why LeBlack was suffering from it.

By the time her son, Tilton, was three years old, her condition had deteriorated so badly that her eyesight had started to be affected (caused by high blood pressure) and she could hardly eat or walk. Her friends took her to hospital.

There she was put on a dialysis machine, which works as an artificial kidney. In people whose kidneys function well, the organs filter the blood, removing waste products (such as urea, sodium and potassium) and excess liquid in the form of urine. One tube carries blood from the arm into the machine where it is "cleansed", then returns it through another tube. Dialysis patients don't pass urine.

"She was lucky," says Dr Rosemarie Baillod, the renal physician who treated LeBlack. "Dialysis was hard to come by in those days. I would get four to six patients a month and a committee would select one for treatment. Patricia's situation was difficult - she was single and she had no support whatsoever. It would have been difficult to turn her away. The idea of keeping someone on long-term dialysis was still new and we didn't know how long people could survive on it for. Then, people thought they wouldn't live - now they expect to. Our early patients knew they were special, that they had been given a chance."

LeBlack says her friends were told that she probably wouldn't live. "It was really bad, I felt really ill. I couldn't eat anything, I couldn't see. I was in fairyland. My friends looked after my son. He was a quiet little boy. He seemed to understand what was going on."

After five months in hospital, her condition became manageable and LeBlack was allowed to go home but her doctors were not sure she could survive for long. They thought she might have 10 years left, at most. "It made no sense to cry about it. I thought, that's OK, my son will be 13. I got myself together," she says very matter-of-factly. But wasn't she upset about it? "Honestly, I wasn't. We don't know what's going to go on in life."

But despite her ready acceptance and brave face, there is a huge list of things that LeBlack has missed out on. Having relationships has been difficult and she hasn't had a long-term partner, or more children; childbirth can be dangerous for women with kidney failure. She has never been on holiday because she has to go into hospital three times a week. She hasn't been able to work and has brought up her son alone with very little money. Her quality of life has been affected in other ways too. She is not free to eat and drink as she pleases - she is not allowed to drink more than two cups of water, or the equivalent, a day because the excess fluid would remain in her body and could put pressure on the heart and lungs. She must also avoid fruit because of its high water content. Nor is she permitted to eat food that contains salt and potassium or much protein. "It is difficult if you go to friends' houses for dinner or go to restaurants. You are allowed one glass of wine at Christmas," she adds.

At first LeBlack had to go back to the hospital three times a week for 10 hours at a time to be attached to the machine. Now that the machines have improved, the sessions are down to three times a week for three-and-a-half-hour sessions. The machine has a timer, which counts down. "I never look at it," she says. "It makes time go slower if you're counting it down."

She was told she might be able to have a transplant but didn't want one. At the time, kidney transplants were new - the Royal Free hospital didn't start them until 1968. Baillod remembers LeBlack being adamant, telling her, "You're not experimenting on me".

One of the most difficult things to deal with, LeBlack says, is the number of people she grew to know over the years who haven't survived. Baillod estimates that 1,000 renal patients have been treated at the Royal Free in the time LeBlack has been going for treatment.

"A lot of them committed suicide because they couldn't cope with it," says LeBlack. "It is difficult for people. But I had my son. I had to pull myself together and whatever life I had left, to make use of it." Does she get frustrated? "No, because I'm not greedy. I'm happy. You have to be patient, you have to be disciplined. I take it seriously, I've never missed a session. I'm an easygoing person. The life is hard but what are you going to do about it? I didn't sit around crying. I don't envy anyone for what they've got. I don't worry about money; it is family and friends that are important. You have to make use of what life you have. I'm just lucky I came to England. I don't know how long I would have survived if this had happened when I was at home."

Baillod, who is now retired, puts LeBlack's survival down to her self-discipline. "She knew what was asked of her in terms of her lifestyle and she knuckled down and did it. She's a real battler."

Despite her upbeat attitude, LeBlack does seem tired. "The machine is keeping me alive but it causes other problems," she says. "My bones ache, I've got arthritis in my hands. But I can't imagine what it's like not to share my life with this machine."

She adds, with a sly smile: "We're very attached".