It is widely accepted that HIV-infected subjects are incapable of transmitting sexually the infection while their plasmatic viral load remains undetectable. In order to assess the percentage of HIV-infected patients showing undetectable viral loads during their antiviral treatment we studied a population of patients regularly assisted at a general hospital. A total of 298 patients (162 men; 54.36%) were admitted to the study. The mean age was (mean ± standard deviation) 47.83 ± 11.69 years and the mean CD4+ cell count was 693.93 ± 363.87 x 106 cells/ml. These variables did not show statistically significant differences between men and women. Plasmatic viral load was undetectable in 230 patients (77.81%). The remaining 68 patients (22.82%) showed a mean of 9856.67 ± 70922 copies/ml. These values were higher in men than in women (17379.39 ± 95521.51 copies/ml vs 895.78 ± 5952.99 copies/ml, respectively; p = 0.015, Student t-test). In line with these findings, CD4+ cell count was significantly lower in men (575.10 ± 345.14 cells/L vs 707.04 ± 373.46 cells/L, respectively; p = 0.0019, Student t-test). 187 out of 231 patients receiving their first antiretroviral treatment showed undetectable viral loads (80,95%), while only 42 out of 67 patients having previously received other antiretroviral schemes had undetectable levels of plasmatic viral load (61,69%; p = 0.002, χ2). These findings show that an important number of patients may keep detectable levels of plasmatic viral load during antiretroviral treatment, being therefore capable of sexually transmitting the infection to their couples.