Cutaneous papilloma have been observed at high prevalences in populations of brown bullheads (Ameiurus nebulosus) and white suckers (Catostomus commersoni) distributed throughout the Great Lakes, with no clear-cut delineation between prevalences in fish populations from industrialized sites in comparison to more pristine sites. However, bullhead papilloma prevalences greater than 20% were never observed at pristine reference sites, while high prevalences of sucker papilloma (> 20%) were only observed in populations from the lower Great Lakes. It is probable that there is a viral etiology for papilloma affecting bullheads and suckers, although chemical contaminants may influence the development of these neoplasms through some mechanism. Hepatic neoplasms of both cholangiocytic and hepatocytic origin have been observed at relatively high prevalences in Great Lakes populations of brown bullheads and white suckers. Prevalences of total hepatic neoplasms of > 9% in bullheads and > 3% in suckers were usually only observed in populations from industrialized areas of the Great Lakes. However, relatively high prevalences of hepatic neoplasms were also observed in suckers and bullheads from pristine areas where there is a preponderance of older fish in the population. In the case of both bullheads and suckers, high prevalences of hepatic neoplasms appear to be associated with severe biliary disease (e.g. cholangiofibrosis, cholangiohepatitis). It is highly probable that the etiology of hepatic cancers in bullheads and suckers from the Great Lakes is associated with exposure to chemical contaminants, and in some areas, specifically with exposure to polynuclear aromatic hydrocarbons. However, other chemical contaminants may indirectly influence hepatic tumor rates by increasing the severity of biliary disease in fish from contaminated areas. Overall, cutaneous papilloma prevalences above approximately 25% and hepatic neoplasm prevalences above approximately 5% in these species should be interpreted as an indicator of environmental degradation in the area where fish were surveyed.