The aim of the present study was to evaluate the impact of obesity and insulin resistance on testosterone formation from androstenedione and its contribution to biochemical hyperandrogenemia in all different phenotypic subgroups of PCOS patients. The case-control study included 1087 PCOS women and 206 regularly menstruating, ovulatory controls. The main clinical measurements included anthropometric and basal hormonal characteristics and evaluation of hyperandrogenic and insulin resistance-related features. The results were the following: In PCOS women with biochemical hyperandrogenemia, obesity significantly lowers serum A levels and increases T to A ratio. These findings were not present in PCOS women with clinical hypeandrogenemia and in normal ovulatory controls.