Background HIV, hepatitis C (HCV), and human T-cell lymphotropic virus I (HTLV-1) are associated with high global burdens of disease, notably in resource-poor locales. They share similar routes of transmission and cause chronic infections with associated morbidity. We performed a cross-sectional study to assess the impact of HTLV-1 infection on clinical outcomes in HIV/HCV-coinfected patients. Methods We enrolled 102 (72.3%) with HIV/HCV coinfection (Group 1) and 39 (27.7%) triply infected with HIV, HCV, and HTLV-1 (Group 2). We reviewed medical records of two groups of patients followed in two outpatients services in Salvador, Brazil. We collected and compared demographic, behavioral-related information, immunologic, virologic, and histologic parameters for HIV-1 and HCV infection. Results Demographics, virologic, and immunologic characteristics were similar in the two groups; a higher proportion of triply infected patients (Group 2) reported any history of injection drug use compared with dually infected (Group 1) patients (75% vs 45.8%; P = 0.003). No differences were seen between groups in HIV clinical outcomes (CD4 count and viral load). Alanine aminotransferase levels were significantly higher in HIV/HCV-coinfected patients (P = 0.045). Liver fibrosis damage based on Metavir scores was similar between groups (0.97) but was worse with lower CD4 cell count (under 200 cells/mm3) (P = 0.01). Conclusions HIV/HTLV-1 and HIV/HCV coinfections may worsen clinical related outcomes, but virologic and immunologic outcomes were similar in both groups. Hepatic measures were worse in patients with more severe immunosuppression.