Insulin-dependent diabetes in children is a heritable disease, although it does not fit into any simple genetic model. Research is not facilitated by the lack of knowledge of the different etiological mechanisms and the inability to classify the disease. Human leucocyte antigens (HLAs) coded for in the major histocompatibility complex on the 6th chromosome have given the most important insights into the genetics of diabetes, and these genes probably provide the majority of genetic susceptibility to type-I diabetes. Several facts indicate that there are at least two loci of importance, one associated with HLA B8, DR3 and another with HLA B15, DR4, which probably are usually included in extended haplotypes in linkage disequilibrium. There seems to be a substantial DNA polymorphism and further studies may detect fragments that are even more strongly associated with diabetes than the DR type. A combination of more detailed immunological, perhaps virological or other environmental investigations on diabetic patients and their families, together with the use of specific DNA probes for the HLA region, might reveal the susceptibility genes.