Diabetic ketoacidosis is the most serious complication at the onset of type 1 diabetes mellitus (DM). In Germany, population-based data on its occurrence at DM onset are not yet available. In a population-based study in a North Rhine-Westphalian region, Germany, during 1993-95, data on the clinical presentation at type 1 DM onset were obtained from hospital records for 262 patients under 15 years of age (81% of eligible patients). Information on social status was obtained from 148 families by a standardized questionnaire. The most frequently reported symptoms were polyuria (93.9%), fatigue (64.2%) and weight loss (59.4%). Mean duration of symptoms was 3.5 weeks. At diagnosis 18.3% of the children presented impaired consciousness and 3.5% coma. Mean glucose level was 25.1 mmol/l. Severe ketoacidosis (pH < or = 7.2) was present in 16.0% of the children. Metabolic derangement was more severe in children under 5 years. Low social status was significantly associated with increased risk of severe ketoacidosis (OR = 3.54, 95% CI: 1.10-11.35). Frequency of ketoacidosis at DM onset needs to be reduced through increased public and medical awareness of the presenting characteristics of childhood DM.