HLA haplotype distribution was analyzed in nuclear families of patients with insulin-dependent diabetes mellitus. Sixteen families with two or more diabetic siblings were examined, a total of 69 subjects, 33 of these diabetic siblings and 36 normal subjects (siblings and parents). The data were processed using the involved sibling pairs method based on a mixed model making use of a conditional probability approach. The ratio of diabetic sibling pairs concordant by 2 haplotypes, 1 haplotype, and discordant by 2 haplotypes was 9:5:2 vs. 1:2:1 expected according to Mendel's accidental distribution (p < 0.025). Increased incidence of siblings concordant by 2 haplotypes proves the presence in the HLA domain of one or several genes responsible for the development of diabetes mellitus. Siblings identical by two HLA haplotypes with the diabetic proband are at a higher risk of developing this disease.