Serum complements, C3, C4, and C3 proactivator (C3PA), were evaluated prospectively for their diagnostic power in detecting hepatocellular carcinoma (HCC) in patients with liver cirrhosis (LC). Sixty-three LC patients, including 36 LC with HCC patients, were recruited for this study during the period from March to November 1986. The cutoff values of the complements were set according to retrospective study including 17 LC patients and 20 HCC patients. The values with highest accuracy, 75 mg/dl C3, 16 mg/dl C4, and 18 mg/dl C3PA, were selected. Based on these data, the positive predictive values, negative predictive values, and the accuracies of complements were as follows: C3, 88.9%, 85.2%, and 87.3%; C4, 93.8%, 80.6%, and 87.3%; and C3PA, 66.7%, 90.9%, and 70.9%, respectively. The accuracy elevated to 93.7% when C3, C4, and alpha-fetoprotein were combined as the diagnostic criteria. This study supports the use of complement testing as a new diagnostic tool for HCC screening in LC patients and for follow-up evaluation in posttherapy patients of LC with HCC.