Lewis RD, Narayanaswamy AK, Farewell D, et al. - This case‐control study was undertaken to determine if the complement system is activated in polycystic ovary syndrome (PCOS) in the fasting and postprandial state. In 84 women with PCOS and 95 healthy controls, fasting complement levels were measured. Complement activation post‐oral fat tolerance test was contrasted in 40 additional participants (20 PCOS, 20 controls). Activation pathway (C3, C4, C3a(desArg), factor B, factor H, properdin, Factor D) and terminal pathway (C5, C5a, terminal complement complex [TCC]) proteins have been measured by commercial or in‐house assays. In the fasting state as well as postprandially, activation of the complement cascade was evident, involving both the activation (C3, C3(adesArg), factor H) and terminal pathways (C5a(desArg), TCC). In addition, in metabolically unhealthy individuals, complement dysregulation was the most pronounced and associated with both obesity and insulin sensitivity. The authors discovered that activation and terminal complement pathway components are elevated in patients with PCOS, particularly in the presence of insulin resistance and obesity.