Since 1992, prevalence data on HTLV-I and II have been collected as part of an ongoing viral seroprevalence study in clients of six First Nations alcohol and drug treatment centres in British Columbia, Canada. Prior studies indicate that the lifetime risk of clinical disease (neurologic or hematologic) resulting from HTLV-I infection is low (less than 5%) and HTLV-II to date has not been clearly associated with clinical disease. In 1993, the first cases of HTLV-I-associated myelopathy or tropical spastic paraparesis (HAM/TSP) were reported in four Aboriginal residents of British Columbia; these were the first reports of HTLV-I linked disease among Aboriginal persons in Canada. All clients of the treatment centres involved in this study were offered confidential, voluntary testing following pre-test counseling, and the results are given to participants before the residential session is complete. 1953 men and women were tested; 11 were positive for HTLV-1 (0.56%) and 33 were positive for HTLV-2 (1.8%).