Aflatoxins are well-known animal hepatocarcinogens, but the association between aflatoxins and human hepatocellular carcinoma remains to be elucidated. A study method consisting of indirect immunofluorescence assay combined with densitometry was developed to quantitate aflatoxin B1 DNA adducts in smeared liver tissue obtained at the time of biopsy for diagnosis in 50 hepatocellular carcinoma patients in Taiwan. Monoclonal antibody 6A10, generated against the persistent form of the major N7 guanine adduct of aflatoxin B1, was used for detection of adduct. Thirty-five (70%) of the hepatocellular carcinoma samples had detectable levels of aflatoxin B1 DNA adducts (⩾ 1/106 nucleotides). The detection rate was slightly lower in men (69%) than in women (75%), and younger patients had a significantly higher rate of adducts (83%) than did older ones (58%). Carriers of both HBsAg and HBeAg, carriers of HBsAg only and noncarriers had different rates of detection: 29%, 74% and 82%, respectively. Patients with family histories of hepatocellular carcinoma had a higher detection rate (100%) than did those patients without such histories (67%). No association was found between aflatoxin B1 DNA adducts in liver tissue and Child's score for severity of liver disease. The results suggest that aflatoxin B1 may be involved in the pathogenesis of hepatocellular carcinoma in Taiwan. Our immunohistochemical method for analysis of adducts in small numbers of cells from the target organ should improve results of monitoring for the biologically effective dose of aflatoxin. HEPATOLOGY 1992;16:1150–1155.