OBJECTIVE 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.

RESEARCH DESIGN AND METHODS 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.

RESULTS At least one parent had diabetes in 18.6% of the families of younger-onset individuals and in 38.6% of the families of older-onset individuals. For those of younger-onset diabetes, 9.1% of fathers, 8.3% of mothers, and 1.3% of both parents had diabetes; the corresponding percentage for those of older-onset diabetes were 11.5, 23.5, and 3.6%, respectively. The difference between frequencies in mothers and fathers was significant (P < 0.0001) 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% and 7.7 and 9.8% 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% CI, 0.72–2.05); if the mother had diabetes, the RR was 2.39 (95% CI, 1.64–3.48); and, if both parents had diabetes, the RR was 5.61 (95% CI, 3.37–9.34). In the older-onset individuals, the corresponding RR values were 1.69 (95% CI, 1.35–2.13) for fathers, 1.72 (95% CI, 1.44–2.06) for mothers, and 2.42 (95% CI, 1.81–3.25) for both parents.

CONCLUSIONS 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.