Multivariate models for the risk of insulin-dependent diabetes mellitus for siblings of patients with insulin-dependent diabetes were developed using logistic regression analysis. The individuals studied in this report are full siblings of the insulin-dependent diabetic patients diagnosed at Children's Hospital of Pittsburgh between 1964 and 1982. For all siblings considered together, the sharing of two (but not one) HLA haplotypes, the presence of insulin-dependent diabetes in a parent, and being relatively young at the time the proband in the family was diagnosed were significantly associated with increased risk. On the other hand, B7 + siblings had a significantly decreased risk compared to B7--siblings, indicating the presence of an HLA-linked protective gene for the development of the disease. There was a significant interaction between sharing two HLA haplotypes and maternal age at the time of birth; for non-HLA-identical siblings, risk increased with increased maternal age, but maternal age had little or no effect on the risk for HLA-identical siblings. When non-HLA-identical siblings (0 or 1 HLA haplotypes shared) were analyzed separately, only the presence of insulin-dependent diabetes in one of the parents and increased maternal age at the time of birth of the sibling were found to be significantly associated with increased risk. Both of the totally non-HLA-identical diabetic siblings (neither HLA haplotype shared) in this data set had a parent with insulin-dependent diabetes, indicating that at least one HLA haplotype must be inherited in common with an affected family member for diabetes to develop. For HLA-identical siblings, the significant variables were the age of the sibling when the proband was diagnosed, the possession of B7, and maternal age at the time of birth of the sibling, and all three were negatively associated with risk.