During a prospective, 24-mo case-controlled study, 551 patients from northeastern Thailand were independently evaluated for Opisthorchis viverrini infection, hepatobiliary tract disease, and hepatic carcinoma to determine whether there was any association between hepatic carcinoma and O. viverrini infection. Stool examination by the formalin-ether concentration method revealed O. viverrini ova in 389 (70.6%) patients. Of the 551 patients, 72 (13.1%) had both clinical and laboratory evidence of hepatobiliary tract disease, chronic liver disease, or hepatic carcinoma, alone or in combination. Of these 72 patients, 28 (38.9%) had a liver biopsy that revealed cholangiocarcinoma in 7 patients with O. viverrini ova in their stools, and in 4 patients without. In another patient with ova in the stool combined hepatocellular carcinoma and cholangiocarcinoma was found. In the 4 patients with cholangiocarcinoma who had no O. viverrini ova in their stools, ova were detected in the bile fluid aspirated from the intrahepatic biliary tree during exploratory laparotomy. An additional patient with clinically suspected cholangiocarcinoma and O. viverrini ova in stool had a left supraclavicular lymph node biopsy specimen taken that revealed metastatic adenocarcinoma; this adenocarcinoma was interpreted as compatible with cholangiocarcinoma. Cholangiocarcinoma, therefore, was found only in patients with O. viverrini ova in stool or in the intrahepatic biliary tree. Statistical analysis revealed that patients with known O. viverrini infection had a higher incidence of cholangiocarcinoma than did patients without such infection (X2 test, p less than 0.05).