It is generally accepted that oxidative stress is involved in HIV infection. However, the role in oxidative balance of Highly Active Antiretroviral Therapy (HAART) is still debated. In our study we assessed serum oxidant and antioxidant levels in an HIV-1-infected population treated with HAART, and compared them with those of untreated HIV-1 patients and HIV-1-negative subjects. The study included 116 HIV-1-infected patients (86 HAART-treated and 30 untreated), and 46 HIV-negative controls. Serum oxidant levels were significantly higher in the HIV-1 treated group as compared to untreated and control groups. In addition, a decrease of serum total antioxidant status was observed in the HIV-1 treated group. To be noted is that patients who rigorously follow antiretroviral therapy (optimal HAART adherence) have significantly higher oxidative status than those who do not closely follow the therapy (poor HAART adherence). Analysis of variance revealed no significant further increase in oxidative status in HIV-1-infected patients taking antiretroviral and other drugs with the exception of psychiatric drugs (e.g. anxiolytics or antidepressants). Taken together, our results indicate that HAART may affect oxidative stress in HIV-1-infected patients and suggest that antiretroviral therapy plays an important role in the synergy of HIV infection and oxidative stress.