In this study, we report type 1 diabetes age-of-onset association with HLA class II (DRB1, DQB1, and DPB1) and class I (A) genes in 222 multiplex families from the Human Biological Data Interchange. Linear regression showed a small (R2 = 0.26) but significant correlation in the ages of onset among sib pairs. A strong association in age of onset between members of sib pairs was observed when the analysis was performed using contingency tables that split sibs into three age-at-onset ranges (0-10, 11-20, and 21-36 years). The association is strongest for sib pairs that share both haplotypes and is nonsignificant for sib pairs that do not share any DR-DQ haplotypes. A goodness-of-fit test revealed that DR-DQ haplotype sharing cannot account for all the association in age of onset among sib pairs. The age-of-onset distribution of DR-DQ haplotypes is affected by the DPB1 and A alleles present. The strongest associations were found with the A locus: DR3/DR4 genotype frequency decreases with age of onset in this data set only among A*0101- individuals, and A*2402 is strongly associated with younger age of onset in many DR-DQ haplotypes.