The incidence of IDDM in the age group over 30 years was estimated in a historical prospective study, using clinical and biochemical measurements at onset as criteria for classification. The study population, nearly one million, represents 20% of the Danish population. The degree of ascertainment was > 99%. One thousand two hundred and forty patients were treated with insulin during the study period (1973-77). Based on the clinical and biochemical variables, the patients were classified into three groups: insulin-dependent diabetes mellitus (IDDM) accounted for 16.2%, insulin-treated diabetes mellitus for 54.1% and short-term treated diabetes mellitus for 29.6% of the total insulin-treated group. The incidence of IDDM in the age group over 30 years at onset was 8.2 100,000(-1) year-1. The cumulative incidence rate (0-90 years) was 1.5-1.6 per cent. The present study indicates that IDDM may develop at any age. Thus the life-time risk of developing IDDM is higher than hitherto expected.