PURPOSE: Circulating micro-ribonucleic acids (miRNAs) are small noncoding RNA molecules that influence gene transcription. We conducted the present profiling study to characterize the expression of circulating miRNAs in lean and obese patients with polycystic ovary syndrome (PCOS), the most common endocrine and metabolic disorder in premenopausal women. BASIC PROCEDURES: We selected 11 control women, 12 patients with PCOS and 12 men so that they were similar in terms of body mass index. Five control women, 6 men and 6 patients with PCOS had normal weight whereas 6 subjects per group were obese. We used miRCURY LNA™ Universal RT microRNA PCR for miRNA profiling. MAIN FINDINGS: The expression of 38 miRNAs and was different between subjects with PCOS and male and female controls. The differences in 15 miRNAs followed a pattern suggestive of androgenization characterized by expression levels that were similar in patients with PCOS and men but were different compared with those of control women. The expression of 13 miRNAs in women with PCOS was similar to that of control women and different compared with the expression observed in men, suggesting sexual dimorphism and, lastly, we observed 5 miRNAs that were expressed differently in women with PCOS compared with both men and control women, suggesting a specific abnormality in expression associated with the syndrome. Obesity interacted with the differences in several of these miRNAs, and the expression levels of many of them correlated with the hirsutism score, sex hormones and/or indexes of obesity, adiposity and metabolic dysfunction. PRINCIPAL CONCLUSIONS: The present results suggest that several serum miRNAs are influenced by PCOS, sex hormones and obesity. Our findings may guide the targeted search of miRNAs as clinically relevant markers for PCOS and its association with obesity and metabolic dysfunction in future studies.