The goal of this study was to investigate clinical outcomes and survival probabilities among persons coinfected with human immunodeficiency virus (HIV) and human T lymphotropic viruses types 1 and 2 (HTLV-I/II). A nonconcurrent cohort study of 1033 HIV-infected individuals was also conducted. Sixty-two patients were coinfected with HTLV-I, and 141 patients were coinfected with HTLV-II. HTLV-I/II coinfection was highly associated with African-American race/ethnicity, age of >36 years, higher CD4(+) T cell count at baseline and over time, and history of injection drug use. Coinfected patients were more likely to have neurologic complications, thrombocytopenia, respiratory and urinary tract infections, and hepatitis C. Despite having higher CD4(+) T cell counts over time, there was no difference in the incidence of opportunistic infections. Progression to both acquired immunodeficiency syndrome (AIDS; adjusted hazard ratio [aHR], 0.50; 95% confidence interval [CI], 0.25-0.98) and death (aHR, 0.57, 95% CI, 0.37-0.89) were slower among HTLV-II-coinfected patients, compared with time-entry- and CD4(+) T cell count-matched control subjects. In conclusion, HIV-HTLV-I/II coinfection may result in improved survival and delayed progression to AIDS, but this happens at the expense of an increased frequency of other of clinical complications.