To avoid, delay, or ameliorate the microvascular complications of diabetes, intensive insulin therapy with the goal of normoglycemia is required. In most patients with insulin-dependent diabetes mellitus (IDDM), this cannot be achieved because of severe hypoglycemia, which is one of the major causes of morbidity in diabetic patients. To assess the frequency and characteristics of severe hypoglycemia in a single diabetologist's practice, we surveyed 211 consecutive patients with IDDM for a history of severe hypoglycemia (SH), defined as events requiring the assistance of another person. Of these 211 patients, 135 (64%) had at some time had SH. Those with SH had a longer duration of IDDM, currently took more insulin injections, had a higher prevalence of neuropathy and nephropathy, and were less likely to be using human insulin. No difference was found in age, average glycosylated hemoglobin level, frequency of home glucose monitoring, or presence of retinopathy. Half of the patients with SH had confusion and were treated with glucose orally. The remainder were in coma and received i.v. glucose or IM glucagon. Perceived causes of SH were lack of food, excessive insulin, and unusual exercise. We conclude that the frequency of SH is underestimated in clinical practice. Risk factors for SH are longer duration of IDDM, presence of neuropathy and nephropathy, and use of animal insulin. Better education in avoidance and treatment of SH would reduce the morbidity of SH.