Abnormalities of insulin secretion rather than insulin sensitivity are described in women fulfilling the American criteria for gestational diabetes. We examined insulin secretion and insulin sensitivity in 38 women at risk of gestational diabetes categorized according to the less stringent WHO criteria, based on the 75 g oral glucose tolerance test, performed at 24 weeks gestation. Insulin sensitivity was assessed at 28 and 36 weeks using the short insulin tolerance test. Applying WHO criteria, 18 women had GDM. Age and body mass index of the GDM and glucose tolerant women were similar (32.4 +/- 1.1 (SE) vs 32.3 +/- 1.9 yr; 28.7 +/- 1.5 vs 28.8 +/- 1.7 kg m-2, respectively). Fasting glucose was higher in the GDM women than controls (5.1 +/- 0.2 vs 4.5 +/- 0.1 mmol l-1, p < 0.025) while fasting insulin was similar (75 +/- 18 vs 90 +/- 16 pmol l-1). The 30-min insulin concentration during the OGTT was lower in the GDM women than controls (436 +/- 61 vs 788 +/- 152 pmol l-1, p < 0.05), while the insulin sensitivity at 28 (87 +/- 5 vs 76 +/- 5 mumol l-1 min) and 36 weeks (73 +/- 8 vs 76 +/- 8 mumol l-1 min) was similar. A negative correlation existed between the 30-min insulin and 120-min glucose concentration during the OGTT (Rho -0.328, p < 0.05). The WHO criteria for GDM identify women with similar abnormalities of insulin secretion as the more stringent American criteria.