Sera from 714 mentally retarded carriers of hepatitis B surface antigen were screened for alpha fetoprotein (AFP) by monoclonal radioimmunoassay. Serum AFP levels were less than 20 mcg/L in 708 (99.2%) carriers. One 29-year-old carrier with normal liver function had serum AFP level of 1500 mcg/L, which increased to 12,500 mcg/L after 72 days. She died of multifocal hepatocellular carcinoma (HCC) with cirrhosis. Five other carriers with serum AFP levels between 20 and 165 mcg/L are alive without clinical HCC. No correlation was found between serum AFP level and race, age, sex, Down's syndrome, serum alanine aminotransferase level, and hepatitis B e antigen positivity. Single cross-sectional serum AFP screening by itself is not sufficient for early diagnosis of HCC.