A total of 113 patients with infection due to human T-cell leukemia virus type 1 (HTLV-I) were evaluated at the University of Miami from January 1988 to March 1993. Forty patients were identified with adult T-cell leukemia/lymphoma (ATLL) and 63 with HTLV-I-associated myelopathy (HAM). Three had concomitant ATLL and HAM. Two HAM patients co-infected with human immunodeficiency virus type 1 (HIV-I) developed clonal lymphoproliferative disease during the study period. Patients with ATLL have a poor prognosis; multiple chemotherapy regimens including high-dose cytotoxic agents have been utilized with a small impact on survival. Most of our patients are currently treated with experimental regimens. Rheumatologic or autoimmune illnesses were identified, mostly in HAM patients, and a small number developed immunodeficiencies in the absence of other definable etiologic factors. Most of the patients were immigrants from areas of endemicity in the Caribbean basin, although many Americans were also recognized. HTLV-I/II infection was diagnosed serologically and typed as HTLV-I by polymerase chain reaction (PCR) or a modified Western blot when a DNA sample was not available. In 24 of 40 patients with ATLL, Southern blot hybridization performed on DNA extracted from peripheral blood lymphocytes or tumor tissue demonstrated clonal HTLV-I integration. In South Florida, ATLL and HAM are now seen frequently. Since HTLV-I infection is associated with a 4% lifetime risk of developing ATLL and an additional 0.25% lifetime risk for developing HAM, a large pool of asymptomatically infected individuals must exist here.(ABSTRACT TRUNCATED AT 250 WORDS)