A personal series of 6780 patients with diabetes mellitus is reported. Of these 1410 were thought to have insulin-dependent (Type 1) diabetes and 4926 non-insulin-dependent (Type 2) diabetes. Among the former, 128 patients were only diagnosed when in severe ketoacidosis or coma. In 116 patients the diabetes was diagnosed in pregnancy. Chronic alcoholism was an aetiological factor in 75 patients; in 52 it led to the diagnosis being made, and it complicated treatment in 129 additional patients. In the patients with Type 2 diabetes whose treatment was stabilized 23.5% were having insulin injections, 44.5% tablets, and 32.0% diet only. Sight-threatening retinopathy developed in 21.3% of patients with Type 1 and 7.9% of those with Type 2 diabetes. The rate of developing sight-threatening retinopathy was 1.1% of patients per year. Blindness occurred in 0.28% of patients with Type 1 diabetes per year and 0.097% per year in Type 2 diabetes. If the mean survival of patients with retinopathy going blind is 7.5 years, this would mean 7500 people in the UK blind from diabetic retinopathy. There was a striking drop in the annual incidence of blindness after 1970 coinciding with the introduction of specific treatment for diabetic retinopathy. Juvenile cataract developed in 1.7% of patients who developed Type 1 diabetes before 30 years of age. Clinically important diabetic neuropathy developed in 17.4% of patients with Type 1 and 11.6% of those with Type 2 diabetes. The main features were paraesthesiae and numbness (49%), neuropathic ulceration (37%), pain (5%), autonomic symptoms (5%), and amyotrophy (4%). Oculomotor palsies and mononeuropathies were noted. Foot ulceration occurred in 81 patients with Type 1 and 279 of those with Type 2 diabetes. Charcot changes in the feet were noted in 21 patients. Major amputations were needed in 18 patients with Type 1 and 60 with Type 2 diabetes. Proteinuria believed to be due to diabetic nephropathy developed in 12.8% of patients with Type 1 and 4.7% of those with Type 2 diabetes. The prevalence of early renal failure was 4.6% and 1.4%, respectively. Coronary artery disease was noted in 9% of patients with Type 1 diabetes, and was more common in those who developed diabetes after 20 years of age. Myocardial infarction was as common in women as in men. In Type 2 diabetes coronary artery disease gave rise to symptoms in 19.1%, and myocardial infarction was more common in men.(ABSTRACT TRUNCATED AT 400 WORDS)