We evaluated the epidemiology of diabetic ketoacidosis in the period 1960-1979. In Frederiksborg County, Denmark, the incidence of ketoacidosis at the county hospital increased from 60 per 100,000 in the period 1943-1963 to 120 per 100,000 in the period 1960-1979. In the investigation period we found an increasing incidence confined to urban areas. Precipitating factors were not somatic in 53% of the cases. Patients in the lowest social class (V) were in a higher risk group, experiencing ketoacidosis more often and having a higher frequency of severe acidosis. Forty-nine percent of the patients have had a diabetes duration of more than 5 yr. The lethality rate decreased from the period 1943-1963 and was 4.7% in the investigation period 1960-1979.