Several recent studies reported the detection of partially deleted HTLV-I provirus in biopsies of lesions from patients with mycosis fungoides (MF) and T-cell anaplastic large-cell lymphoma. We studied lesions from 59 patients (21 B-cell lymphomas: 16 diffuse and five follicular; 11 cutaneous T-cell lymphomas, including seven MF; one T-immunoblastic lymphoma; 10 diffuse anaplastic large-cell lymphomas: two B, four T, and four of indeterminate phenotype; three Hodgkin's lymphomas; eight atypical lymphoid proliferations; four other lymphoid lesions, and one squamous-cell carcinoma) using primers to the gag, pol and pX regions of HTLV-I in the polymerase chain reaction (PCR) to detect relevant sequences. A total of 10 patients showed one or more PCR-amplifiable products, including five of 11 patients with cutaneous T-cell lymphomas (45%) as compared with one of 21 patients with B-cell lymphomas (4.3%). We did not find a high incidence of positivity in anaplastic large-cell lymphomas, as reported previously. Detectable HTLV-I sequences were not limited to any subtype of lymphoma, and a pX sequence was detected in a squamous-cell carcinoma. Sequence analysis of one amplified product from each of the three regions studied showed a 94.2, 100, and 98.9% homology to the corresponding prototypical gag, pol, and pX HTLV-I sequences, respectively, indicating that the amplified sequences were derived from HTLV-I or a very closely related virus. HTLV-I sequences were detected in a significant proportion of patients with cutaneous T-cell lymphoma, but their role in the pathogenesis of the neoplasm is still unclear.