Not every HTLV-1 seropositive carrier will become a HAM/TSP patient. Fewer than 5% will exhibit neurological dysfunction or, eventually, hematological malignancy such as adult T-cell leukemia/lymphoma, suggesting that other host or viral factors are responsible for disease onset.

HAM/TSP is usually a progressive neurological disorder but it is rarely fatal. Most patients live for several decades after the diagnosis. Their prognosis improves if they take steps to prevent urinary tract infection and skin sore formation, and if they enroll in physical and occupational therapy programs.