Type 1 insulin-dependent, diabetes mellitus (Type 1 DM) is thought to be more prevalent in individuals with Down's syndrome. To ascertain the local prevalence of Type 1 DM in patients with Down's syndrome in a geographically defined area, the four diabetes clinics in Lothian were surveyed and 13 patients with Down's syndrome and Type 1 DM were identified. Using data from previous epidemiological surveys which determined the prevalence of Down's syndrome in the general population, the prevalence rate of Type 1 DM in patients with Down's syndrome was calculated to be between 1.4 and 10.6%, a prevalence considerably higher than in the general population. Although 7 (54%) of the Down's syndrome patients were treated with once daily administration of insulin, the mean HbA1c value of the group was similar to that observed in a control group of 39 age-, sex- and duration-matched Type 1 patients, all of whom were taking two or more injections of insulin daily. Glycaemic control was therefore of similar quality to matched Type 1 patients without Down's syndrome, despite the frequent use of simple insulin regimens, which may relate to the more stable lifestyle of these patients.