These data show that, in non-obese women with PCOS that did not differ for insulin resistance and inflammation compared to controls, proteins in the alternative complement activation pathway (C3, iC3, and properdin) were elevated in women with PCOS in comparison with the matched controls; C4, belonging to the classical complement activation pathway, was also increased in PCOS. The gene discussed is C4A; the disease is polycystic ovary syndrome.