Serum levels of monoamine oxidase (SMO) have been determined in 16 normal adults, in 76 patients with hepatic diseases and in 44 patients with non-hepatic diseases. Elevated enzyme levels were found in patients with cirrhosis and in those with chronic hepatitis or liver cancer associated with moderate or advanced fibrosis. It was also observed that levels of SMO increased gradually with progression of hepatic fibrosis. The increased enzyme concentration could not be ascribed to hepatocellular necrosis or cholestasis. In non-hepatic diseases, elevated levels of SMO were observed in the patients with acromegaly, hyperthyroidism and prooxidase gressive systemic sclerosis, in which biosynthesis and degradation of collagen were considered to be enhanced by overproduction of hormones or by unknown causes. The determination of SMO concentration is useful as a supplemental diagnostic procedure for organ fibrosis, especially for advanced hepatic fibrosis.