The human T-cell leukemia/lymphoma virus (HTLV) is a novel type-C retrovirus isolated from patients with T-lymphoproliferative malignancies. Thirteen cases of HTLV-associated malignancy from US centers were studied in detail. Ten of these cases share common clinical features and define a typical virus-associated adult T-cell leukemia/lymphoma (ATL). All ten patients presented with Ann Arbor stage IV lymphoma because of skin involvement, bone marrow involvement, or lymphomatous leptomeningitis. Lymphadenopathy occurred in 7 of 10 patients at presentation, and the malignant cells were cytologically pleomorphic. Leukemia occurred in 60% of the patients at presentation. Hypercalcemia was found initially in two-thirds of the patients, with lytic bone lesions or positive bone scans in 7 of 10. Complete remission occurred in 40%, but all have relapsed. These cases closely resemble those virus-positive cases of adult T-cell leukemia/lymphoma (ATL) reported from Japan and the Caribbean. Three additional virus- positive patients had atypical presentations and diagnoses (acute lymphocytic leukemia, Sezary's syndrome, leukemic reticuloendotheliosis), usually with less aggressive clinical courses and atypical demographic and laboratory features. Presence of HTLV serum antibodies in cases of ATL (with hypercalcemia and circulating malignant cells) appears to define a distinct clinicopathologic entity that may occur in geographic clusters.