Reactive hypoglycemia (RH), which is a postprandial hypoglycemic state, occurs within 2-5 h after food intake. It is classified as idiopathic, alimentary, or diabetic reactive hypoglycemia. We studied the incidence of reactive hypoglycemia and looked for my correlations between it and the presence of insulin sensitivity and/or beta cell function in young lean polycystic ovary syndrome (PCOS) patients. This study was designed as a cross-sectional study in 64 lean young women with PCOS (BMI <= 25 kg/m(2)), Various indices or insulin sensitivity and beta cell function derived from the oral glucose tolerance test (OGTT) results were used. We found the rate of RH to be 50% in lean young women with PCOS. DHEA-S and PRL levels were found to be lower in subjects with RH (P < 0,05 and P > 0,05. respectively). Beta cell function indices such as the insulinogenic index (at 120 min), CIR (at 120 min) and HOMA beta cell index were found to be insignificantly higher in the RH group than the nonreactive hypoglycemia (NRH) group. The 4 h glucose level. but not the 3 h glucose level, was significantly correlated with insulin resistance indices, such as fasting insulin level, HOMA-IR, Quicky index, and FIRI in the RI I group. Significantly decreased DHEA-S levels were an interesting finding. In conclusion, there is an urgent need to investigate RH in lean young women with PCOS. Our results indicate that more definite insulin resistance occurs in Subjects with RH in the fourth hour of the OGTT than those with RH in the third hour. In addition, RH in the fourth hour together with a low DHEA-S level may be predictive of future diabetes in young women with PCOS even when they are not obese. (c) 2004 Elsevier Ireland Ltd. All rights reserved.