In 139 patients with breast or lung carcinoma, malignant melanoma, and Hodgkin's or non-Hodgkin's lymphoma, JC and BK virus serology (hemagglutination inhibition) and urinary polyomavirus excretion (cytology and immunofluorescence microscopy) were studied. Overall, 18 of 70 patients with paired sera (26%) had titer increases against JC or BK virus, and 11 of 114 patients (10%) had evidence for urinary excretion. The infection rate was highest in patients with Hodgkin's or non-Hodgkin's lymphoma (approximately 40%). Serum HAI antibody titers against JC and BK viruses appeared similar to age-matched controls in each patient group--with the exception of decreased BK titers at diagnosis in patients with resected malignant melanoma and increased JC virus antibody titers at diagnosis in patients with poor-prognosis non-Hodgkin's lymphoma (IC-2). The biologic significance of these observations remains to be determined. Initial antibody titers against JC or BK virus were not of prognostic value for subsequent survival in any of the tested patient groups. Both nonspecific immunotherapy and aggressive, multidrug chemotherapy had surprisingly little effect on serum HAI titers to JC or BK virus. Patients with poor-prognosis non-Hodgkin's lymphoma appear especially suitable for further investigation of JC and BK virus infections. Study of nonbrain, nonurinary-tract tissues may disclose other parenchymal sites of polyomavirus replication in these patients.