Recent data provided by the EURODIAB ACE study group have confirmed wide variation in the incidence of insulin-dependent diabetes mellitus (IDDM) across Europe. The aim of this report is to compare age-specific incidence and seasonality at clinical onset of IDDM between study regions. Using a uniform methodology, the EURODIAB ACE framework ascertained 3,168 newly-diagnosed cases of IDDM in children under the age of 15 years during 1989-1990. Eighteen percent of the cases were age 0-4 years at diagnosis, 34% were age 5-9 years and 48% were age 10-14 years. Poisson regression analysis suggested that there were highly significant statistical differences in incidence between the three age groups and between the 24 regions. Although incidence rates in the 0-4 year and 5-9 year age groups varied from region to region in a similar fashion, the pattern of variation in the older age group was different. Seasonality of diagnosis conformed to a sinusoidal model with a peak occurring in winter, a feature which was consistently observed in both sexes and in all age groups. However, a statistically significant heterogeneity in the seasonal distribution was present among regions, those in Scandinavia showing the smallest relative amplitude. The first insulin injection was given the same day or the day after diagnosis in 93% of the cases for whom data were available.