The serum alpha-fetoprotein levels of 17 patients with cancers less than or equal to 3 cm in size were studied using radioimmunoassay to determine alpha-fetoprotein response in the early stage of human hepatocellular carcinoma. The levels were normal in 6 patients (35%), and elevated to 645 +/- 1140 micrograms/L (mean +/- SD) in the remaining patients. The levels were not correlated with tumor size. In 10 surgically resected patients, 5 had elevated levels of alpha-fetoprotein that returned to normal after surgery. The levels and tumor sizes were serially observed for 3-26 mo in the remaining 7 patients not surgically treated. In 1 patient alpha-fetoprotein levels were persistently normal and in the other 6 patients, the levels tended to increase with a median doubling time of 60-75 days (range 30-223 days). Despite continued tumor growth, a spontaneous fall in serum alpha-fetoprotein levels was encountered in 4 patients; in 2 patients the levels even fell within the normal range. After this fall, the levels increased drastically in 4 patients. We concluded that in the early stage of hepatocellular carcinoma, serum alpha-fetoprotein level is frequently normal, and thus determination of serum alpha-fetoprotein levels only is not a reliable indicator in the early detection of human hepatocellular carcinoma. A spontaneous fall in the level of alpha-fetoprotein is not an uncommon finding in the early stages of this cancer and cannot be used to rule out the diagnosis of hepatocellular carcinoma.