The role of HTLV in HIV-1 neurologic disease. We performed a serologic survey for antibodies to HTLV-I/II in the course of a longitudinal study of the neurologic complications of HIV-1 infection. Nine (3.7%) of 242 HIV-1 seropositive subjects and none of 60 HIV-1 seronegative control subjects had antibodies to HTLV-I/II by ELISA. Western blot and polymerase chain reaction confirmed the presence of HTLV-I in 2 subjects and HTLV-II infection in 2 others. Both HIV-1/HTLV-I coinfected subjects and 1 HIV-1/HTLV-II coinfected subject had a slowly progressive myelopathy clinically identical tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). The presence of a myelopathy resembling TSP/HAM in the coinfected subjects suggests that HIV-1 may enhance the expression of neurologic disease caused by HTLV. Patients with a progressive myelopathy occurring in association with HIV-1 infection should be serologically tested for the presence of HTLV. Establishing dual infection has therapeutic and prognostic import as 1 of the HIV-1/HTLV-I subjects substantially improved with corticosteroids and the HIV-1/HTLV-II subject with myelopathy had a marked improvement in the absence of therapeutic intervention. 