Serum alpha-fetoprotein levels were measured by radioimmunoassay in 473 patients with biopsy-proved noneoplastic hepatic disorders; 22% had values greater than 40 ng/ml, whereas only 1 of 350 patients with nonhepatic benign diseases had a value greater than this. Levels exceeded 40 ng/ml in more than 30% of patients with various types of hepatitis, and in 0% to 15% with inactive postnecrotic cirrhosis, primary biliary cirrhosis, biliary tract obstruction, and alcoholic liver disease. Values greater than 500 mg/ml were observed solely in viral subacute hepatic necrois. Only one patient had a level exceeding 3,000 ng/ml, the concentration at which alpha-fetoprotein is detectable by agar-gel diffusion. Of 75 patients with hepatoma, serum alpha-fetoprotein levels exceeded 40 ng/ml in 69%, and exceeded 3,000 ng/ml in 48%. These studies indicate that serum alpha-fetoprotein levels are elevated in several nonneoplastic hepatic disorders when a sensitive assay is used; this phenomenon may reflect hepatic regeneration.