The lateral line in fish is a specialized mechanosensitive system distributed over the entire body, composed of neuromasts, which plays a critical role in discrimination of motion external to the fish. We have documented an epizootic of inflammation and neoplasia selectively affecting the lateral line system of angler caught lake trout (Salvelinus namaycush) in four Finger Lakes in New York from 1985 to 1994. Tumors occurred in 8% (5/64) of mature and 21% (3/14) of immature lake trout in the most severely affected lake. Lesions consist of one or more neoplasms in association with multifocal erosions and ulcerations of epidermis along the lateral line. Lesions vary by age presenting in 2-year-old lake trout as locally extensive intense lymphocytic infiltrates, in 2- to 3- year olds as multiple, variable sized, white foci up to 3mm diameter, and in fish over 5 years old, as multiple white, cerebriform masses greater than 1 cm diameter. Histological diagnoses on the tumors were predominantly sarcomas. Prevalence estimates are statisticlly similar between males and females, and do not vary with season. The cause of this epizootic remains unclear. Disease transmission attempts, virus isolation procedures, and electron microscopic study of lesions have failed to reveal evidence of a viral etiology. The Finger Lakes in which the disease occurs do not receive substantially more chemical pollution than unaffected lakes in the same chain. In addition, the species and tissue specificity of the disease also suggest that a potent environmental carcinogen is not a likely cause, since affected lakes all contain several unaffected salmonid species. A hereditary component may be involved as all affected fish are of the Seneca Lake strain.