The present study combines segregation and linkage information on 30 families ascertained through a proband and a first degree relative affected with insulin-dependent diabetes mellitus (IDDM). An autosomal dominant model with incomplete penetrance was much more likely to fit the family data than a recessive model, whether or not linkage to HLA was assumed. The lod scores for linkage to HLA were 2.46 at θM= θF. = 0.00 for dominant and 1.45 at θM = θF= 0.22 for a recessive model. The results are discussed in light of heterogeneity in likelihoods and lod scores when the families are grouped by familial types, which indicate that the increase in likelihood of a dominant hypothesis can be attributed to the parent-child families and not the sib-sib families. © 1983 S. Karger AG, Basel.