HL-A typing of 150 patients who had developed diabetes mellitus by the age of 30 years showed a significant association with HL-A 8 and W 15. The HL-A genotypes were determined in 17 families in which two or more siblings had this type of diabetes. The zygotic assortment of HL-A haplotypes was found to be significantly disturbed from the expected random pattern, with a reduction in the number of siblings showing no identical haplotypes and an appreciable increase in the number with both haplotypes identical. This appears to be most consistent with the presence of a gene or genes pre-disposing to this type of diabetes at a locus closely linked to the HL-A chromosomal loci. This locus appears to have a fundamental role in the susceptibility to juvenile diabetes.