The incidence and prevalence of depression in diabetic patients in the United Kingdom is unknown. Since depression may influence blood glucose control which in turn may be related to the development of diabetic complications, it is important to estimate its prevalence in diabetic patients. The prevalence of depression was investigated in a group of Caucasian and West Indian, insulin-(IDDM) and non-insulin-dependent (NIDDM) adult diabetics and a non-diabetic comparison group. Prevalence of depression was 8.5% for both groups and a further 19.2% and 14.6%, respectively, had borderline depression. Presence of depression was unrelated to sex, ethnic group, duration or type (IDDM or NIDDM) of diabetes and social class but significantly related to type of accommodation, marital status, and amount of social contact. A higher percentage of diabetics with psychiatric symptoms had one or more current complications compared to 'normal' diabetics. Diabetics suffer from a similar amount of depression to non-diabetics, but psychiatric symptoms may be related to the frequency of diabetic complications.