A case-control, prospective, open-label, clinical trial to evaluate efficacy and safety of a combined zidovudine/Thymus Humoral Factor Gamma-2 (THF) therapy in HIV-infected subjects was conducted in 13 patients. Twenty-six patients were included as controls receiving only zidovudine. The two groups of patients were matched according to sex, age, CDC stage of HIV infection, number of CD4+ T cells and type of previous opportunistic infections (if any) and all patients and controls were naive for antiretroviral therapy at the moment they entered the trial. The observation period was protracted up to 47 months (mean 28 +/- 13 months). No significant difference was observed between the two groups as far as surrogate markers of HIV disease progression are concerned. However, patients receiving zidovudine and THF showed a lower number of opportunistic complications. Only one patient in this group progressed to manifest AIDS while 9 of 18 controls presented disease progression. Four patients died in the case group, all of them were CDC stage IV at admission, and 15 of 26 died in the control group (all CDC stage IV at admission, and four patients who presented disease progression during the study period). Survival time was increased in the case group. The exact immunological effect of thymus hormones in HIV infection has still to be elucidated, but a possible therapeutic role of these agents is foreseeable.