One hundred and thirty-three insulin-dependent diabetic (IDD) patients were genotyped for HLA-A, B, C, DR and 123 of them for Bf. The study shows a relationship between these genes and the age of onset of diabetes and emphasizes the possible heterogeneity of the disease. When patients under the age of 0 years at the onset of the disease were compared with those aged 11 to 29 years, a significant excess of HLA-B18 (41% versus 15%, p less than 0.001) and BfF1 (40% versus 21%, P less than 0.05) was observed. The haplotype B18, BfF1 was also more frequent in the first group of patients (haplotype frequency 18% versus 6%, p less than 0.02). The frequency of the whole haplotype Cw5, B18, BfF1, DR3 and of its segment BfF1, DR3, and the strength of the gametic associations between these alleles were much higher in IDD patients than in non-diabetic controls, irrespective of the age of onset of their diabetes. The association between early age of onset of IDD and the B18, BfF1 haplotype independently of DR3 (no association between age and DR3) suggests that a factor influencing the onset of the disease in young children could be under the control of (1) gene (s) in linkage disequilibrium with B18 and/or BfF1.