Summary Nuclear families of non-insulin-dependent diabetic (NIDDM) patients are uncommon, as usually one or both parents have died. In order to aid identification of complete nuclear families, we have ascertained the disease process at a younger age by studying subjects with previous gestational diabetes. One hundred women who had had gestational diabetes, age (+ SD) 38 (6) years, were screened by fasting plasma glucose (fpg). Sixty-one were found to have either fasting hyperglycaemia (5.5 _< fpg < 7.8 mmol/1) or diabetes. Of these women 35 had both parents alive and the parents of 14 of these women agreed to the assessment of their metabolism by a continuous infusion of glucose with model assessment (CIGMA). Seven probands had impaired glucose tolerance (IGT) and seven were diabetic. They were age 35 (4) years and had body mass index (BMI) 26 (5) kg/m 2. The parents were aged 62 (6) years and had BMI 29 (6) kg/m 2 and their affection status was defined as presence of glucose intolerance (fpg or post-infusion achieved plasma glucose level > 2 SD of an age and obesity matched population). In the 14 families, five probands (36%) had neither parent affected, six (43 %) had one parent affected and three (21%) had both parents affected. Only three probands had a parent with diabetes as defined by World Health Organisation criteria. We concludes that the study of women who have had gestational diabetes allows detection of probands with diabetes or impaired glucose tolerance, who have both parents available for study. A substantial proportion had neither parent affected with impaired glucose tolerance or diabetes, similar to the nuclear families of NIDDM patients. The results are in accord with studies of nuclear families of NIDDM patients in suggesting polygenic inheritance or environmental influences rather than autosomal dominant inheritance with high penetrance. [Diabetologia (1995) 25 years, more than 50 % of the women develop diabetes [1]. Approximately 30 % of women who present with NIDDM in middle age have previously been suspected to have diabetes in pregnancy because of hyperglycaemia or glycosuria [2]. Women who have had gestational diabetes present an opportunity to study diabetes at an early stage in its evolution [3]. They are often overweight, and have both impaired beta-cell function and increased insulin resistance [4,5]. These are similar defects to those found in the first degree relatives of NIDDM patients [5][6][7].Studies of inheritance of NIDDM have been restricted because of the difficulty in identifying two