This study aimed to assess the prevalence of familial aggregation of insulin-dependent diabetes mellitus (IDDM), among Danish families with a diabetic child and to compare epidemiological data for familial and sporadic cases of IDDM children. All IDDM patients aged 19 years or less treated at paediatric departments or departments of internal medicine were identified and asked to complete a questionnaire regarding diabetes onset and family history. Of 1574 probands identified, 1419 participated (90.2%). Additional cases of IDDM were found in 12.8% of the families. Among these families, in 6.8% the father and in 2.1% the mother were diabetics and in 5.0% at least one of the siblings were diabetics. In familial cases the proband was significantly younger at diabetes onset, the parents were younger at birth of the IDDM child and no differences in gender were observed in contrast to sporadic cases, where more males were found. Thus, heterogeneity in epidemiological characteristics was observed between familial and sporadic cases.