In the diagnosis of hepatocellular carcinoma (HCC), serum des-gamma-carboxyprothrombin (DCP) is a useful tumor marker. The conventional immunoassays for measurement of DCP levels, however, are not sensitive enough to detect small HCC. Therefore, we intended to elevate the minimal detection level of DCP by a modified enzyme linked immunosorbent assay method.This modified assay method is similar to the conventional enzyme linked immunosorbent assay (ELISA) method, but the first reaction occurs overnight. As a result, the minimal detection levels of DCP varied from 0.06 AU/ml, by the conventional method, to 0.008 AU/ml, by the modified method. Two hundred and twenty five serum samples from 100 patients with HCC, 75 with liver cirrhosis and 50 with chronic hepatitis were subjected to the present study. Simultaneous determinations of serum DCP by the modified assay and a-fetoprotein (AFP) levels were performed.Eighty five of 100 patients with HCC had increased DCP levels of more than 0.008 AU/ml. This method yielded a sensitivity of 85%, a specificity of 90% and a total accuracy value of 88%. In 27 patients with small HCC (less than 30 mm in diameter), 12 had elevated DCP levels, resulting in a sensitivity of 44%. When the modified DCP assay together with AFP measurement (more than 20 ng/ml) were introduced, the sensitivity was 67% in the 27 patients with small HCC.This modified ELISA method increased the sensitivity in patients with small HCC, and the combination assay of serum DCP and AFP levels was more useful for the early diagnosis of HCC.