To elucidate the mechanism of metabolic adaptation of women with polycystic ovary syndrome (PCOS) during pregnancy, the endocrino-metabolic features of a group of PCOS patients with or without gestational diabetes were studied longitudinally during the three trimesters of gestation. Oral glucose tolerance test (OGTT, 100 g) and hyperinsulinaemic-euglycaemic clamp were performed throughout the study. Plasma concentrations of insulin and glucose were determined by radioimmunoassay and glucose oxidase technique, respectively. Five of 13 PCOS patients developed gestational diabetes (GD) at the third trimester (PCOS-GD), while the other eight patients did not develop any alteration of glucose metabolism (PCOS-nGD). Both fasting glucose and insulin plasma concentrations did not change significantly during pregnancy and no difference was seen between the two groups. On the contrary PCOS-GD group early exhibited higher values of area under the curve (AUC) for glucose and insulin response to OGTT with respect to those found in PCOS-nGD group. This difference was already significant in the first gestational trimester. Moreover insulin sensitivity value (M) was significantly lower in the first trimester of gestation in PCOS-GD as compared with that found in PCOS-nGD group. However, as gestation proceeded, M value decreased in PCOS-nDG group and the difference from PCOS patients developing gestational diabetes was not sustained into the second and third trimesters. Both groups had similar body mass index values and AUC insulin increase from first to third trimester of gestation. It is concluded that early alteration of insulin sensitivity and secretion constitute specific risk factors in PCOS patients for the development of abnormalities of glucose tolerance.