The aim of this study was to determine anti HTLV-I antibody titers in seropositive symptomatic and asymptomatic infected subjects. One hundred seven infected subjects (47 with spastic paraparesis and 60 asymptomatic) were studied. HTLV-I antibodies were determined using indirect immunofluorescence in cells infected with the retrovirus. The mean titer was 1/234 in asymptomatic subjects and 1/2138 in symptomatic patients (p < 0.001). These results suggest an association between HTLV-I antibody titers and clinical stage of infected subjects.