In these non-obese, non-insulin-resistant women with PCOS, there were significant elevations in the levels of alternative pathway complement proteins C3 (p <0.002), iC3b (p <0.02), and properdin (p <0.006), while C4 in the classical pathway was increased (p <0.05); C4a and C4b did not differ between groups. This evidence concerns the gene C3 and polycystic ovary syndrome.