Background: Tropical spastic paraparesis (TSP) is an endemic disease in Chile. In most countries, only 50% of patients are seropositive to HTLV-I. However, new studies suggest that seronegative TSP is also associated with HTLV-I. Aim: To describe cLinical and virological features of seronegative patients with TSP. Patients and methods: Seventy two Chilean patients with TSP, studied by cLinical, radiological and laboratory methods during 1998, are reported. The determination of antibodies to HTLV-I was accompLished by ELiSA, immunofluorescence and Western-blot analysis. Polymerase chain reaction for tax and 5'Ltr genes was made using primers SK 43-44, LTR1 and LTR6. Results: Thirty one patients were HTLV-I positive and 41 were negative. No cLinical, radiological or laboratory differences were observed between both groups. In seropositive patients, tax and 5'ltr viral gene sequences of the HTLV-I provirus were detected in DNA of peripheral blood mononuclear cells. In seronegative cases, sequences of tax gene were detected, exclusively, in 18 of 41 patients. Conclusions: These results confirm an association with HTLV-I infection in 43,9% of the TSP seronegative patients. These findings support the hypothesis that a defective provirus infects peripheral blood mononuclear cells in seronegative cases of TSP. The importance tax gene in the diagnosis of the TSP is also emphasized.