OBJECTIVE: To investigate possible effects of insulin-sensitizing treatment on cortisol metabolism in insulin-resistant patients with polycystic ovary syndrome (PCOS). DESIGN: Randomized placebo-controlled study. SETTING: Academic tertiary care medical center. PATIENT(S): Thirty insulin-resistant PCOS patients. INTERVENTION(S): Sixteen weeks of pioglitazone (30 mg/day) or placebo treatment. MAIN OUTCOME MEASURE(S): Twenty-four-hour 20 min integrated blood sampling for measurement of cortisol and 24 h urinary excretion of steroid metabolites. Relative 5alpha-reductase activity was evaluated by allotetrahydrocortisol (alloTHF)/THF and androsterone/etiocholanolone (A/E) ratios. Delta values denoted changes during the treatment period (16 weeks--basal). Pyridostigmine growth hormone (GH) stimulation tests were performed, and testosterone (T), dihydrotestosterone (DHT), DHEA, DHEAS, adiponectin, and insulin-like growth factor I (IGF-I) were measured before and after intervention. RESULT(S): Insulin sensitivity, GH, adiponectin, and IGF-I significantly increased during pioglitazone treatment, whereas alloTHF/THF levels significantly decreased. Delta alloTHF/THF levels inversely correlated with Delta adiponectin levels. Delta A/E ratio inversely correlated with Delta IGF-I and Delta peak GH during GH stimulation tests. No significant changes were measured in T, DHT, DHEA, DHEAS, 24 h mean cortisol, or urinary excretion of steroid metabolites. CONCLUSION(S): Pioglitazone decreased relative 5alpha-reductase activity, whereas no significant changes were measured in cortisol levels or urinary cortisol excretion.