Atlantic tomcod, Microgadus tomcod, from the Hudson River, New York exhibited a very high incidence of hepatocellular carcinoma while less industrially impacted rivers such as the Pawcatuck River, Rhode Island and the Saco River, Maine were much lower. A comparative pathological study revealed that hepatic function was compromised by fatty infiltration, basophilic foci, areas of cellular alteration, and hepatocellular carcinoma in samples from Hudson River fish. Age and size may also be affected in Hudson River tomcod. An etiology involving xenobiotics is highly suspect for these effects since it is known that elevated detoxification of some organics results in increased hepatic lipid levels. Juvenile tomcod removed from the Hudson after six months and reared in clean water showed no incidence of liver disease six months later.