The relationship between DNA synthesis activity in hepatocytes from cirrhotic tissue and development of hepatocellular carcinoma was studied in 33 posthepatitic patients with Child's grade A cirrhosis. DNA synthesis activity was measured by a bromodeoxyuridine (a thymidine analogue) labeling index, using the bromodeoxyuridine-antibromodeoxyuridine in vitro method, and the patients were followed up prospectively with frequent liver ultrasonography for 2 years. During the 2-year follow-up, 11 of the 33 cirrhotic patients developed hepatocellular carcinoma; they included 8 of the 15 patients (53%) in the high labeling index (greater than 1.5%) group compared with only 3 of the 18 patients (17%) in the low labeling index (less than 1.5) group (P less than 0.05). Five of the latter 18 subsequently had increased synthesis activity. Of these 20 patients who showed high synthesis activities either initially or subsequently, 10 (50%) developed hepatocellular carcinoma, in contrast to 1 of 13 (8%) with persistently low activities (P less than 0.05). Thus, hepatocellular carcinoma seems to develop or may become detectable when DNA synthesis in the background cirrhosis is increasing or remains high.