Reports of tumors in wild freshwater fish populations have been increasing. The highly industrialized nature of the Great Lakes basin suggests that chemically-induced tumors could occur in Great Lakes fish. Several indices of tumors in these fish are reviewed, including: thyroid hyperplasia, gonadal neoplasm, papillomas, skin tumors, and tumors of the liver. The results of this study show that papillomas in white sucker, gonadal tumors in carp x goldfish hybrids, and hyperplasia in coho salmon are widespread in the Great Lakes. Tumor incidence is proposed as a sensitive indicator of the level of carcinogenic compounds in the environment.