Tropical spastic paraparesis (TSP) is a slowly progressive myelopathy associated with increased serum and cerebrospinal fluid antibodies to the human T-lymphotropic retrovirus type I (HTLV-I) (ref. 1), and has been observed in many regions of the world^2–4. A similar condition known as HTLV-I-associated myelopathy⁵ occurs in the Kagoshima prefecture of Japan. Recent^6,7 but controversial^8–10 reports suggest involvement of virus related to HTLV-I in multiple sclerosis. Magnetic resonance imag-ing and electrophysiological studies indicate that TSP lesions are like multiple sclerosis¹¹ in that they are disseminated throughout the nervous system. Complete virus from patients with TSP has proved difficult to isolate^12,13 using techniques successful in adult T-cell leukaemia cases associated with HTLV-I. Here we report the isolation of an HTLV-I-like virus from T-cell lines derived from the peripheral blood and cerebrospinal fluid of TSP patients. The monoclonal antibody OKT3 was used to generate non-trans-formed T-cell lines that express HTLV-I antigens. Infectious virus was demonstrated by co-cultivation and complete, replicating virions were visualized ultrastructurally.