Oral glucose tolerance test (OGTT) is the most commonly used method to evaluate insulin resistance (IR) in the clinical practice. Our objective was to evaluate the diagnostic utility of fasting tests compared with OGTT tests in women with PCO, and the ability of fasting tests to detect postprandial hyperglycemia. One hundred fourteen women with PCO and 29 normal women were evaluated by a 2 hours OGTT. Fasting plasma insulin (INS) and glucose were measured during the test. GLU:INS ratio (r) (fasting glucose/fasting insulin), HOMA (homeostatic model assessment), QUICKI (quantitative insulin sensitivity check index) (fasting tests), as well as the AUCI (area under the curve of INS) and ISI composite (ISI) (insulin sensitivity index) (OGTT tests), were determined. A significant correlation between fasting tests and OGTT tests was found. Normal fasting tests with abnormal OGTT tests were found in 9 patients. No patient with fasting insulin levels less than 9.9 Ul/ml were IR, and all women with fasting insulin levels over 18.4 UI/ml were classified as having IR. We found glucose levels 120 min post glucose load (G 120) > or = 140 mg/dl in 14 patients (12.2%). Fasting glucose and insulin levels and the fasting tests, were poor predictors of impaired glucose tolerance (IGT) and type 2 diabetes (DBT 2). Thus, fasting tests are useful in the diagnostic of IR in PCO patients. OGTT is necessary when the fasting insulin levels range between 9.9 and 18.4 Ul/ml. Women with PCO should undergo periodic screening for abnormal glucose tolerance.