The occurrence of hepatitis B surface antigen (HBs Ag) was investigated in 46 cases of hepatocellular carcinoma (HCC) diagnosed in Vitória, Espírito Santo State, Brazil. 2. Twenty cases were patients in which the HCC was diagnosed by biopsy, laparoscopy or on autopsy from 1977 to 1980. HBs Ag was measured in the serum by reverse passive hemagglutination (13 cases) and radioimmunoassay (7 cases). Twelve cases (60%) were HBs Ag positive. 3. The presence of HBs Ag was studied in 26 cases diagnosed on autopsy using paraffin-embedded liver sections stained with orcein. Eleven cases (42%) showed orcein granules in hepatocytes surrounding the tumor. In only one case were orcein-positive granules detected in the tumor cells. 4. The 50% incidence of HBs Ag among patients with HCC is higher than the occurrence of HBs Ag in the general population of the State of Espírito Santo, i.e. 3.5% among 1300 blood donors to the University Hospital. 5. These data provide additional evidence for the hypothesis that there is a relationship between the hepatitis B virus and the etiology of HCC.