The epidemiology of uncontrolled diabetes mellitus was studied in an 11.2% sample of the Danish population (574,696 inhabitants) during a 24-month period. Some 175 admissions in ketoacidosis (heavy ketonuria and plasma bicarbonate below 21 mmol/l) were recorded. Based on prevalence rates from a socio-economically and ethnically comparable Danish county, the annual incidence rate was calculated to be 0.045 per diabetic. The incidence rate of moderate and severe episodes (bicarbonate less than 16 mmol/l) was 0.032 and of severe episodes only (bicarbonate less than 10 mmol/l) 0.017 per diabetic. The major risk group was female teenagers. The total annual frequency of recurrence was 8.7%: 48% of the male episodes were ketoacidosis (DKA) associated with onset of diabetes, against 30% of the female episodes (P = 0.02). All Danish diabetics were at the time of the survey (1978-79) treated with conventional insulin treatment. Annual incidence rate in these established diabetics was 0.028, i.e. three to five times less than reported during treatment with continuous subcutaneous insulin infusion. Mortality of DKA was low, 3.4%, and dependent upon age and precipitating factor but not upon the degree of acidosis. The overall annual mortality rate was 1.5 per 100 diabetics.