Although early reports of an etiological role of HTLV-I in AIDS are not supported by subsequent epidemiologic and biologic data, all AIDS specialists should be aware of the possible infection of AIDS risk group members with the HTLV viruses. In the United States, HTLV-II and HIV co-exist in high prevalence among IVDU, and coinfected IVDU may progress more quickly to AIDS. The health effects of solitary HTLV-II infection are currently unknown. In Africa and the Caribbean, where HTLV-I is endemic, the epidemic of HIV infection will likewise produce coinfection and possibly faster progression to AIDS. In addition, adult T-cell leukemia/lymphoma and HTLV-associated myelopathy (HAM/TSP) may occur in persons infected with HTLV-I alone. Clinical care of HTLV infected patients is difficult because of societal confusion of these viruses with HIV. The full health effects of both HTLV-I and HTLV-II are not yet known, and even the two known disease outcomes of HTLV-I infection occur in only a few percent of those infected. Without prognostic indicators, and with the long latency from infection to disease, counseling and medical follow-up of patients is difficult. Further research into the clinical effects of HTLV-I and HTLV-II is urgently needed.