We studied the prevalence of antibodies against HTLV-1 among every HIV-infected outpatients assisted in our hospital between January 1st 2000 and June 30th 2003. We reviewed the epidemiological data, clinical findings, viral load and CD4 cells-count, comparing coinfected with non HTLV-1 coinfected. We found a prevalence of HTLV-1 infection of 8.1% (23/282); 8.5% (12/141) in men and 7.8% (11/141) in women [[OR=0.91 (0.36<OR<2.3) p=0.8]; 12.4% (16/129) among patients users of illicit drugs and 4.6% (7/152) among non-users [OR=2.93 (1.09<OR<8.17) p=0.03]; 20.8% (16/77) in injecting drugs users (IDU) and 3.4% (7/205) in non-IDU patients [p=0.000006 OR=7.42 (2.71<OR<20.9)]. Ninety six patients (96/282) had history of AIDS defining diseases (ADD). No differences in the ADD were found among coinfected and no coinfected patients. No patient developed any disease relative to HTLV-1. The mean of CD4-cells among HTLV-1 coinfected with history of ADD naïve patients (n=7) was 211 cells/ml, and 87.9 cells/ml among those not-coinfected (n=55) [t-test=2.82; p=0.006]. The viral load was similar among every investigated group of patients. We found a high prevalence of HTLV-1 infection in HIV-infected patients (higher among IDU). The CD4-cell count of patients suffering from an AIDS defining disease was higher among HTLV-1/HIV coinfected patients than in singly HIV infected, this could show a grade of missfunction of CD4-cells in coinfected patients.