The characteristics of familial Type I (insulin-dependent) diabetes mellitus-that is Type I diabetes in a first degree relative were investigated for children diagnosed before the age of 15 years using data from an international network of population-based registries (the EURODIAB ACE network) and from a case-control study (EURODIAB ACE Substudy 2) conducted by eight of the network's centres. Ecological analysis across the 18 centres showed a positive association between the population incidence rate of Type I diabetes and the prevalence of Type I diabetes in fathers of affected children (Spearman's rank correlation coefficient rs = 0.70, p < 0.001). A similar association was observed with the prevalence in sibling (rs = 0.71, p < 0.001), but the association with prevalence in mothers was weaker and not significant. Pooling results from all centres showed that a greater proportion of fathers (3.4%) of affected children had Type I diabetes than mothers (1.8%) giving a risk ratio of 1.8 (95% CI 1.4 to 2.5). Affected girls were more likely to have a father with Type I diabetes than affected boys (odds ratio 1.56, 95% CI 1.07 to 2.27), but there was no evidence of a similar finding for mothers or siblings. Children with disease onset in the 0-4 year age-range were more likely to have an affected father than were children who were older at onset, and similar although weaker associations were seen in mothers and siblings. This suggests that familial Type I diabetes patients have a younger age at onset than non-familial patients. In conclusion, a positive association between the prevalence of familial Type I diabetes and the population Type I diabetes incidence rate was shown and the characteristics of familial Type I diabetes (younger age at onset and preferential transmission of disease from father to child and particularly from father to daughter) were described.