C3 and neoplasm: We verified that high C3 deposition in GC tissues (odds ratio [OR], 1.848; 95%CI, 1.015–3.363; P = 0.045), along with advanced tumor stages (stage III and IV; OR, 2.609; 95%, 1.725–4.194; P < 0.001), depleted plasma C3 level (< 0.75 mg/ml; OR, 1.801; 95%CI, 1.049–3.090; P = 0.033) and any morbidities after surgery (OR, 2.770; 95%, 1.446–5.305; P = 0.002), were independent factors for poor 5-year OS in GC patients.