Diabetes mellitus is the second most prevalent chronic disease in children in the U.S. It is associated with severe manifestations which include blindness and circulation deficiencies as well as markedly increased risk of death. The etiology of diabetes mellitus remains a mystery although both genetic and environmental factors have been implicated. The geneticist is confronted with a number of obstacles in his attempts to unravel this problem, including differences in the definition of affected individuals. This matter was certainly clarified by the separation of noninsulin-dependent diabetes (NIDDM) and insulin-dependent diabetes mellitus (IDDM) into two separate disease entities. Twin studies, however, show that IDDM cannot be entirely due to genetic causes as concordance is no more than about 50%. Although the disease is then clearly not inherited per se, the "susceptibility" to diabetes seems almost surely inherited and, provided this susceptibility, the disease can be brought on by environmental factors. Until the underlying mechanism causing IDDM is completely ascertained, we have to rely on genetic markers to approach the study of the inheritance thereof. Since, in the early 1970s, research by Nerup's and Cudworth's groups revealed associations between the HLA-B locus and IDDM, the HLA markers are considered the classical genetic markers for IDDM susceptibility. In this paper, we review the nature of the genetic susceptibility to IDDM and the possible environmental factors which can bring on the disease.