We have measured the serum erythropoietin concentrations in 14 patients with liver cirrhosis and in 14 patients with a hepatocellular carcinoma. Among these patients, 2 with liver cirrhosis (14.3%) and 7 with a hepatocellular carcinoma (50.0%) were found to have raised serum erythropoietin concentrations, ranging up to 40 mU/ml. Negative correlations were found between erythropoietin and the RBC, and the Hb and Ht in the cases with liver cirrhosis. In contrast, a positive correlation which was not significant was found only between the erythropoietin and the RBC in cases involving a hepatocellular carcinoma. This has suggested that the relationship between the erythropoietin and the RBC in cases of a hepatocellular carcinoma differs from the relationship seen under the usual physiological circumstances of those with liver cirrhosis.