A survey to determine the prevalence of diabetes in people with Down's syndrome was carried out at t raining centres in the West Riding of Yorkshire. Out of a total of 914 people known to the County Health Department as having this condition, 456 attended either junior or adult training centres and, of these, 404 provided urine specimens on two separate occasions. There were seven cases of diabetes among this group—a prevalence of 17 per thousand. Of more significance is the finding that the rate among children aged 0–14 years was 20·6 per thousand, which is considerably higher than expected in a population of this age group. It is postulated that there is an underlying defect of carbohydrate metabolism in people with Down's syndrome.