Early detection of hepatocellular cancer (HCC), makes it surgically resectable with a potential for cure. The test most commonly used to detect HCC is the measurement of serum alpha-fetoprotein (AFP) levels. However, the AFP test is negative in HCC detection in more than 30% of the cases. Riboflavin carrier protein (RCP) is a growth and developmental protein, synthesized and secreted by the liver and hence was of interest to measure its levels in HCC. A prospective double blind evaluation of RCP levels in serum from 93 subjects was undertaken. These included 22 proven cases of HCC, 25 normal controls, 20 cases of alcoholic hepatitis, 20 cirrhotics, and 6 cases of primary biliary cirrhosis (PBC). RCP was measured by a sensitive and specific radioimmunoassay (RIA). RCP was immunohistochemically localized in paraffin sections of liver specimens using standard methods. Mean serum RCP levels in HCC were 21.75+/-14.66ng/ml and were significantly higher (p<0.0001) than those in normal controls (0.73+/-0.25ng/ml), alcoholic hepatitis (1.92+/-0.82ng/ml), PBC (2.16+/-0.74ng/ml), or cirrhosis (5.02+/-1.52ng/ml). Serum RCP levels were elevated in all 22 HCC cases. In contrast serum AFP levels were elevated in 11 of 22 HCC cases. Immunohistochemical analyses revealed positive staining for RCP in liver tumors. We have previously demonstrated elevation of serum RCP levels in breast adenocarcinoma. Our results suggest that serum RCP levels are significantly elevated in HCC also and could potentially serve as a marker for HCC detection under conditions where breast cancer is ruled out. In combination with AFP, serum RCP levels have the potential of serving as a panel of markers for better detection of HCC.