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To evaluate the relative frequency of parental history of diabetes in a population-based study of younger- and older-onset groups of individuals with diabetes and a comparison group of individuals without diabetes.Study participants were queried about a family history of diabetes. The frequencies of positive responses for parents and siblings were compared between younger- and older-onset groups.At least one parent had diabetes in 18.6 percent of the families of younger-onset individuals and in 38.6 percent of the families of older-onset individuals. For those of younger-onset diabetes, 9.1 percent of fathers, 8.3 percent of mothers, and 1.3 percent of both parents had diabetes; the corresponding percentages for those of older-onset diabetes were 11.5, 23.5, and 3.6 percent, respectively. The difference between frequencies in mothers and fathers was significant (P less than 0.001) in the older-onset group. In the nondiabetic comparison group, for those of similar ages to the younger- and older-onset groups, the corresponding frequencies were 6.2 and 9.0 percent and 7.7 and 9.8 percent for fathers and mothers, respectively. The greater frequencies of diabetes in mothers of older-onset diabetic individuals were not accounted for by maternal age. In younger-onset individuals, the relative risk (RR) of diabetes in a sibling if the father had diabetes was 1.22 (95 percent CI, 0.72-2.05); if the mother had diabetes, the RR was 2.39 (95 percent CI, 1.64-3.48); and, if both parents had diabetes, the RR was 5.61 (95 percent CI, 3.37-9.34). In the older-onset individuals, the corresponding RR values were 1.69 (95 percent CI, 1.35-2.13) for fathers, 1.72 (95 percent CI, 1.44-2.06) for mothers, and 2.42 (95 percent CI, 1.81-3.25) for both parents.These data confirm a familial influence on the frequency of diabetes. The excess of cases in mothers of older-onset diabetic individuals is compatible with both environmental and genetic influences.