Analysis of HLA haplotypes occurring in more than one, only one, or no diabetics in GAW5 multiplex insulin-dependent diabetes mellitus (IDDM) families suggested: 1) DR3, DR4, DRw6, and DRw8 are positively associated, and DR2 is negatively associated, with IDDM; 2) DR4 haplotypes are more diabetogenic than DR3 haplotypes; some DR3 haplotypes lacking B8/B18 are more diabetogenic than those carrying B8/B18; DR3 haplotypes with DR beta TaqI bands [2,5,10/11] are more diabetogenic than those without; DR4 haplotypes that carry DQw3.2 are more diabetogenic than those that do not; some DR2 haplotypes are diabetogenic rather than protective. Analysis of DR3 and DR4 transmission from DR3/X and DR4wX (X not equal to 3,4) healthy parents suggested: 3) no distortion of transmission to healthy children and 4) mothers transmit DR4 (and perhaps DR3) less often than fathers to diabetic children. Analysis of INS, GM, and HLA haplotype sharing in affected sib pairs suggested: 5) random segregation of INS haplotypes and 6) a tendency to increased sharing of GM haplotypes in affected sib pairs who were HLA-identical and DR3/4, compared with pairs who were not.