BACKGROUND Polymerase chain reaction (PCR) is a new diagnostic procedure which has been used to recognise HIV-infected individuals who remain seronegatives. METHODS Genomic DNA isolated from the peripheral blood mononuclear cells of 90 high-risk individuals were analyzed by PCR using gag primers SK 38/39. Subjects were classified in four groups: 42 drug abusers, 35 heterosexual partners of HIV-infected individuals, 9 homosexual men, and 4 health care workers accidentally exposed to HIV. Liquid hybridization using radiolabelled probes was done to confirm the results. All samples were also tested for HIV antigen and antibodies (Ab) using EIA and Western blot (WB). RESULTS Two out of 11 (18%) drug abusers and 5 out of 34 (14%) couples were PCR positive in absence of HIV antibodies. This silent HIV infections were not recognized in homosexuals and health care workers. All 38 seropositive samples were PCR positive. None of the samples PCR+/Ab- was positive for HIV antigenemia or showed indeterminate results in the WB assay. CONCLUSIONS Silent HIV infections were recognized in drug abusers (18%) and couples of HIV-infected individuals (14%). Mechanisms involved in the production of this "occult" HIV infections are reviewed.