Human T-cell lymphotropic virus type I (HTLV-I) associated adult T-cell leukemia/lymphoma (ATLL) occurs endemically in southwestern Japan, the Caribbean, and West Africa, but occurs sporadically in most of the rest of the world. However, because ATLL and non-HTLV-I associated T-cell neoplasms share overlapping clinicopathologic features, the prevalence of ATLL in nonendemic regions is unknown. In this study, 75 T-cell neoplasms randomly procured from the metropolitan New York City area were examined by polymerase chain reaction (PCR) for the presence of integrated HTLV-I proviral sequences. HTLV-I genomic sequences were detected by PCR in 6 of the 75 cases (8%); this result was confirmed by Southern blot hybridization. The clinicopathologic features of the HTLV- I positive and HTLV-I negative T-cell neoplasms were then compared. Although the clinicopathologic features of patients from these two groups overlapped, some findings were more commonly associated with HTLV-I positive neoplasms. Five of the six patients with HTLV-I positive neoplasms were from HTLV-I endemic areas, five were black, five were women, and five were less than 45 years of age, while the majority of the patients with HTLV-I negative T-cell malignancies were elderly white men. The incidence of hypercalcemia and lytic bone lesions was significantly more common among patients with HTLV-I positive T-cell neoplasms (P less than .001 and P = .004, respectively). The immunophenotypes of the HTLV-I positive and negative tumors were similar; however, all HTLV-I positive neoplasms were CD7 negative (P less than .001). In summary, our findings: (1) demonstrate the special clinicopathologic and immunophenotypic features of HTLV-I positive T-cell neoplasms, (2) suggest that most of the rare cases of HTLV-I-associated T-cell neoplasms occurring in HTLV-I nonendemic areas are actually endemic cases; and (3) that PCR is a sensitive, clinically useful technique for identifying HTLV-I associated T-cell neoplasms.