I have had problems being a psychiatrist since the first day I worked in the Maudsley hospital. The Maudsley is in south London, and every day I saw more and more young people of African and Caribbean origin developing serious mental illnesses. As a young black man I identified with them, found it disturbing, and vowed to do something about it: so I started research in the area. Unfortunately, that made things worse rather than better; partly because the statistics are so chilling, and partly because of the response to them.

Over the last 30 years there have been 20 studies showing that people of Caribbean and African origin have an increased risk of being treated for serious mental illnesses such as schizophrenia and mania. The increased rate is of epidemic proportions - between five and 12 times greater than for white people. And if anything, it is getting worse.

On March 31 each year, a one-day census is carried out for all Britain's psychiatric inpatients, and the results of last year's census have just been published. Of the 32,000 people in hospital, those who defined themselves as black Caribbean and black African were over-represented by three- or fourfold. But one other group stood out - those who defined themselves as "black other". The vast majority of this group are young, British-born black people, and they were 18 times more likely to be in hospital than the British average.

It is always prudent to treat statistics with caution. Hospital admission reflects not only the amount of illness in a community but also the ability of the community to cope with that illness. For instance, in highly supportive, tight-knit communities, more people are treated at home. But it is hard to believe that this increased rate is not at least in part due to a true increase in the amount of illness. This is backed up by years of research and a recent international review, which concluded that migrants are more likely to develop mental illness. But the risk is doubled in black migrants to white countries, and the risk is increased again in their children. It seems that it is not about migration alone or being black - it is about being black in a white country. The rate of serious mental illness in the Caribbean and in Africa is not high, but the rate of mental illness in Britons of Caribbean and African origin is.

You can recover from a psychotic illness, but many people have long-term problems. The cost to the individual with a psychotic illness, to their family and carers and wider society, is immense. Most sufferers are unemployed and on benefit; there is an increased risk of suicide; life expectancy is lower; and their children are more likely to develop a mental illness and be taken into care. In one study, 50% of carers were clinically depressed. This undermines their ability to support others.

Psychotic illnesses start young and persist. Taking the cost of benefits and loss of tax revenue into account, mental illness is the single most expensive part of the health budget. Those of African and Caribbean origin are already disproportionately living in poverty and struggling to offer a social system that supports and develops the young. How does a community stop itself from disintegrating under such a burden?

We have some of the best mental health services in the world but we are nowhere near a cure for psychosis. Where there is no cure, prevention is important, and where there is an increased rate of illness in a group they should be the target for prevention. But we have no prevention strategy.

We have an excellent plan for improving mental health services for black and minority ethnic groups, but we need to go further. If we knew that one group in society were 10 times more likely to develop lung cancer, we would focus on them - perhaps with a targeted anti-smoking strategy. We would not just make lung cancer treatment services more equitable.

Though we will not be able to prevent all psychosis, we should be able to prevent some of it. We know that psychotic illnesses are associated with poverty, poor education, racism, living in a city, poor obstetric care, head injuries or brain infection when you are young, childhood trauma, family break-up, and cannabis use. We know that targeting childhood and adolescence is important.

Prevention of mental illness in black communities is the sort of complex problem that should attract a high-level government inquiry that leads to action. I am used to hearing politicians say that doing nothing is not an option. This is an area where the phrase has real meaning. The high rates of mental illness in people of Caribbean and African origin are not going to go away. If anything, their legacy will blight a generation and the impact will be felt by us all.

Mental health services have been accused of institutional racism over their treatment of black patients. The government has asserted that this is not a useful term. But the lack of a coherent prevention strategy is an institutional problem that needs institutions, not individuals, to act.

I have moved from south London to north London but, 17 years on, I am still watching young people of Caribbean and African origin coming through the door with serious mental illnesses which tear their families apart. We are still doing nothing to stop it.

· Kwame McKenzie is professor of mental health and society at the University of Central Lancashire and a senior lecturer in transcultural psychiatry at University College London
k.mckenzie@medsch.ucl.ac.uk