A significantly decreased familial diabetes aggregation on the maternal side of insulin-dependent diabetic children born between 1974 and 1984 as compared to those born between 1960 and 1973 was observed. This phenomenon was explained by an increased prevention of hyperglycaemia during pregnancy since 1974, particularly of gestational diabetes in women with familial diabetes aggregation. A highly significant predominance of Type II diabetes was found in great-grandmothers of infantile-onset diabetics on the maternal side as compared to the paternal side. This finding may suggest that gestational diabetes, which can represent a risk factor for diabetes transmission on the maternal side, is often followed by "extragestational" type II diabetes in higher age. This conclusion is supported by the data of familial diabetes aggregation obtained in insulin-dependent infantile- or juvenile-onset diabetics, when familial diabetes aggregation was ascertained as late as at least 20 years after birth of the diabetics. In this case, a significant predominance of familial diabetes aggregation in first- and second-degree relatives, particularly in grandmothers, was found on the maternal side as compared to the paternal side. All these findings are consistent with a teratogenetic effect of gestational diabetes on diabetes susceptibility in the offspring, which can be prevented by avoiding hyperglycaemia in pregnant women and hyperinsulinism in foetuses.