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We studied family histories of diabetes mellitus in the firstdegree relatives of 356 type II diabetic propositi in whose families monogamy is practiced. Positive family histories were noted in 32% of the propositi: parental 20%, sibling 14%, and offspring 0.6%. In 11 pedigrees with conjugal diabetic parents, 33% (18/55) of their offspring were diabetic. Paternal influence was significantly higher than maternal influence (43 of 62 vs. 19 of 62, Z = 2.86, p < 0.01). The presence or absence of sibling history did not depend on the body mass index in the propositi. We also studied pedigrees of 10 propositi in whose paternal families polygamy is practiced. In these families also a trend toward greater paternal influence was noted. We conclude that (1) a family history of diabetes is present in one-third of diabetic propositi, (2) paternal influence is stronger than maternal influence in the transmission of diabetes, (3) sibling history for diabetes has no relation to the body mass index of the propositi, (4) prevalence of diabetes is higher in the offspring of conjugal diabetic parents, and (5) studies in polygamous families are a new approach that may help to quantify the genetic load transmitted from a parent when there is heterogeneity in the spouses.