Serum cytokine profiles, T-cell subsets, and general parameters of immune activation were evaluated in 15 patients with acute primary HIV-1 infection, and compared with those obtained from 18 patients with acute primary Epstein-Barr virus (EBV) infection and from 18 control subjects in order to elucidate possible defects of immune response to HIV in early phases of virus-host interaction. Mean CD4+ cell count, serum concentrations of interleukin (IL)-2, IL-4, soluble IL-2 receptor (sIL-2R), tumor necrosis factor (TNF)-alpha, 5'-neopterin, and beta 2-microglobulin were significantly lower in acute HIV-1 infection than in EBV infection. Both acute HIV-1 and EBV infections were characterized by significantly higher mean CD8+ cell count and soluble CD8 antigen (sCD8) levels compared to control subjects, while acute HIV-1 infection was accompanied by the highest interferon (IFN)-gamma serum concentrations. In primary HIV-1 infection, significant impairment of CD4+- mediated T-helper function may lead to viral escape and persistence of infection despite an early and vigorous CD8+ T-lymphocyte activation.