Complex segregation analysis was applied to a sample of 1,293 nuclear families each with at least 1 diabetic patient. The families were divided into 2 groups depending on the proband's treatment: insulin-dependent (IDG) and insulin-independent (IIG). Heterogeneity analysis has revealed a highly significant difference in the IIG group when families were divided into different mating types. The higher recurrence risk was found in the group with affected mothers. Also evidence for a major recessive gene was found in the IIG group, while it was not possible to distinguish between the hypothesis for absence of a major locus and absence of polygenic inheritance in the IDG group. Risks to develop the disease were calculated for a few typical situations.