Patients with malignant lymphoma of the bladder were studied, and three clinical groups were defined: those with primary lymphoma localized in the bladder, lymphoma presenting in the bladder as the first sign of disseminated disease (nonlocalized lymphoma), and recurrent bladder involvement by lymphoma in patients with a history of malignant lymphoma (secondary lymphoma). The differences in these groups regarding lymphoma type, clinical presentation, and clinical outcome were studied. Mayo Clinic Tissue Registry records from 1940 to 1996 were searched to identify patients with lymphomas involving the bladder. The lymphomas were classified based on review of the histology and immunophenotype performed by immunoperoxidase methods. Clinical records were reviewed. Presenting symptoms included urinary frequency, dysuria, hematuria, and lower abdominal and back pain. Primary lymphoma was present in six patients. All were B-cell lineage low-grade lymphomas of the mucosa-associated lymphoid tissue (MALT) type. No patient had recurrent lymphoma or died of lymphoma. Nonlocalized bladder lymphoma occurred in 17 patients; one with low-grade lymphoma of the MALT type, four with follicle center lymphomas, and 12 with large cell lymphomas. Excluding two patients who died postoperatively, median survival was 9 years. Six patients died of lymphoma in the follow-up period. Secondary bladder lymphoma occurred in 13 patients: two with low-grade lymphoma of the MALT type, one with follicle center lymphoma, one with mantle cell lymphoma, and nine with diffuse large cell lymphomas. Median survival in this group was 0.6 years. Low-grade lymphoma of the MALT type was the most frequent type of primary bladder lymphoma and was associated with an excellent prognosis. The bladder can be the presenting site of lymphomatous involvement in patients with more widespread disease. Survival in this group is quite favorable and is presumably dependent on lymphoma histologic type, stage of disease, and other prognostic factors. Bladder involvement by recurrent lymphoma is a sign of widely disseminated disease and is associated with a very poor prognosis.