BACKGROUND: Human T cell lymphotropic virus type 1 (HTLV-1) is associated with tropical spastic paraparesis (TSP). Peru is an area of endemicity for HTLV-1. METHODS: All patients with suspected cases of TSP referred to our institute (Institute of Tropical Medicine Alexander von Humboldt, Lima, Peru) from 1989 through 2002 were interviewed and tested for HTLV-1. All patients with positive results were evaluated by an expert physician. Disease progression was defined as "rapid" if the time between TSP onset and inability to walk unaided was <2 years. RESULTS: Among 165 patients enrolled, the symptoms and signs most frequently found were spasticity (in 97.5% of patients), hyperreflexia (95.4%), lower limb paresthesia (90.2%), pyramidal signs (82.6%), urinary complaints (82.0%), and lumbar pain (79.0%). Rapid progression was present in 21.5% of patients; mean age at TSP onset was higher among these patients than among slow progressors (P<.001). Severe spasticity, diminished vibratory sensation, and tremor were found more frequently among rapid progressors, compared with slow progressors. CONCLUSIONS: HTLV-1--associated TSP is frequently diagnosed in areas of HTLV-1-endemicity. A subgroup of patients experiences rapid disease progression.