Five cases of hepatocellular carcinoma in whom diagnosis was made when the tumor was relatively small, are described. In 2 cases, serum alpha-fetoprotein (AFP) strted to rise sharply, which enabled early detection and surgical removal of the tumor. Serum AFP was below 100 ng per ml, but above the upper normal limit by radioimmunoassay, and was unfluctuating for a considerable period of time before it began to rise in 2 cases. It was negative throughout in 1 case, who lived more than 4 years after the tumor had reached a detectable size. In 4 of 5 cases, the tumor seemed to have evolved during a stage of chronic hepatitis or its transition to cirrhosis. In 1 case with chronic schistosomiasis and advanced mixed macro- and micronodular cirrhosis, a 1.5-cm tumor was detected by celiac angiography. These observations on time relationship of oncogenesis may be generalized to modify the cirrhotic liver. Necessity is emphasized for the early detection of this type of carcinoma to monitor serum AFP in chronic hepatitis patients, particularly in those with unfluctuating, mildly abnormal levels of AFP.