The measurement of somatic cell mutation may assist in the assessment of human cancer risk. The glycophorin A (GPA) assay, which measures the frequency of variant erythrocytes in persons with blood type MN, was used to directly assess in vivo mutability in 30 patients with hepatocellular carcinoma (HCC). HCC patients showed significantly increased frequencies of both hemizygous (MO) and homozygous (MM) variants, due to somatic loss of expression of the N allele, when compared with 27 patients with chronic liver disease and 21 healthy controls. The mean elevations of the MO and MM variant frequencies (VF) in HCC patients were 2-3-fold greater than the comparable VF in the control groups. The mean MO and MM VF in the patients with chronic liver disease was slightly elevated compared to that in healthy controls, but the difference was not significant. In the 19 anti-hepatitis C virus (HCV)-positive patients with a history of blood transfusion, significant linear relations between VF and the duration of HCV infection were observed for MO and MM. These data indicate a high background frequency of somatic mutations in HCC patients. The GPA assay may prove to be a useful estimation of the individual's risk of development of HCC.