Four years ago I gave my sister Paula a kidney. It was just before Christmas and I'm sure we exchanged books, or knitwear or something too, but the details of the gifts we wrapped remain sketchy. The kidney (her kidney, now) is doing fine work filtering impurities from her blood. I don't think she works it as hard as I did, so things have worked out well for both of them.

When she was 21, Paula had an allergic reaction to penicillin that caused her immune system to attack her skin and kidneys. It began with pinprick sensations around her lower legs, and within two days both her ankles and knees had seized up. These unpleasant symptoms were followed by much more serious kidney problems. When she told me that she might need a kidney one day and that I was a potential candidate, I said OK and forgot about it. It was another 10 years before her deteriorating kidneys narrowed her options to either dialysis or a pre-emptive transplant.

I was happy to attend the first, crucial blood test, though I was less than keen on a major operation and the possibility of three months' unpaid convalescence. I wanted to help, but I also wanted my life to continue undisturbed. There was a part of me that hoped I would be ruled out of the donor process, and thought that mum or dad would be better candidates. My reluctance made me ashamed.

The outcome I feared most was the blood tests ruling out my parents, leaving me the only suitable candidate. You're constantly told no one will think any less of you if you say you don't want to donate, but who could refuse when to do so would announce one's unsurpassed selfishness to the world? (I later met a woman who donated a kidney to one of her sisters; a third sister had not only refused to be tested but had encouraged the donor not to be tested either.)

It turned out my parents and I were all good matches. I started to take time to talk to friends about my feelings, doing my best not to sound selfish, even though that's how I felt. I asked one, "What if I say no?" He didn't hesitate, "You won't say no. You'll do it, and it won't be a big deal." He was mostly right.

Over the next month or two, to their bitter disappointment, my parents were both ruled out on medical grounds. My reaction surprised me: I was happy. Now the only candidate, my ambivalence vanished. I felt relieved that neither of my parents would have to undergo a big operation – I'd been so busy agonising about my own tangled feelings that I hadn't thought about them.

Further tests had made it clear that I was a very good match and in great shape to face an operation. One ultrasound operator even told me my candidate kidney was "beautiful". (I bet she says that to all the donors.)

As it faded, I began to understand the roots of my initial reluctance. I was afraid of being forced to decide whether to donate because I was afraid that I might not want to, but wouldn't have the guts to say no. Now my head cleared: I did want to donate. It was a unique opportunity to do something good, and I wanted to spare my sister the risks and rituals of dialysis, and the long wait for a cadaver's kidney.

The operation itself wasn't a big deal. Because I opted for surgery they had to tell me every risk involved: risk of death? One in 5,000 (is that high or low?). Risk of chronic pain for the rest of my life? Depends on whether the surgery is open or keyhole – keyhole lessens the risk (I can't remember the exact figures, but I do remember thinking it might be better to be the one in 5,000). My upper lip wasn't stiff exactly – I had a minor freak-out when the op was delayed for a few days at the last minute – but when Paula and I were finally admitted, we managed to have a laugh. I even got to take my soon-to-be-ex-kidney to Pizza Express for a valedictory beer the evening before the op (my sister is teetotal, so for the donor kidney it was goodbye to me and goodbye to my pre-op stimulant of choice).

There were no nerves the next morning, just a series of procedures. At the last moment, before she was wheeled away, Paula said, "It's hard to say … but thank you," and gave me a hug. I managed to fire back a bright "You're welcome!", which somehow didn't spoil the moment for the orderly who was discreetly wiping his eyes. Paula's surgery was open, mine keyhole. Once she was ready for the transplant they brought me down and placed me in the theatre next door. I was anaesthetised, then the team disconnected my left kidney via two small holes in my abdomen and removed it through a larger hole in my stomach. They then popped next door and gave it to Paula, leaving her two "birth" kidneys in place.

Two days after the transplant my sister felt better than she had in two years. She stayed awake until midnight for the first time in as long as she could remember and even had rosy cheeks. She was taken off her high blood pressure tablets and managed three journeys to the double doors of the ward and back. She was home in a week.

I was home in three days, falling asleep with the unfinished crossword in my lap. Aside from some severe referred pain (irritation in the diaphragm causing terrible shoulder pain for reasons I still fail to understand) my recovery was quick. I could have returned to work in six weeks, but my employer, anticipating a longer convalescence, had hired someone on a three-month contract so told me to enjoy the break. I took a trip to the Lakes and fondly recall running up a hill in a state of giddy excitement.

On the night before our operations, a donor on the ward spent much of the night moaning in pain after his surgery. The next evening he was up and about, telling us he felt "euphoric". He'd donated a kidney to his nine-year-old daughter, who had been on dialysis for a year. He told us he'd woken knowing he'd done a good thing. His joy was deepened when staff told him his daughter was doing well. He stumbled off grinning, his dressing gown flapping open to reveal his shaved stomach and the freshly bandaged scars he bore with obvious pride.

My friend was right – I did it. But he got one thing wrong: it was a big deal. I don't know if I'll ever do a better thing. It was a privilege to be able to help my sister live a longer, happier life. She is doing well. I am very lucky.