We searched for the presence of human T-cell leukemia virus type I (HTLV-I) sequences in central nervous system and muscle lesions of 3 patients with tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM) and 3 patients with HTLV-I-associated polymyositis. Proviral DNA coding for the Tax protein was found by polymerase chain reaction amplification in DNA extracted from lesions of every patient with TSP/HAM or HTLV-I-associated polymyositis. In contrast, viral RNA was found occasionally by in situ hybridization in muscle lesions of some patients with polymyositis, but was never found in central nervous system lesions of TSP/HAM patients.