BACKGROUND Unprotected sex can lead to transmission of the human immunodeficiency virus-1 (HIV-1) to the spouse of an infected individual. We studied the incidence of HIV-1 infection in the spouses of cases diagnosed to have HIV-1 infection by serology and the polymerase chain reaction (PCR). METHODS Blood samples collected from 9 index cases and their respective spouses were tested for HIV-1 infection by ELISA, Western blot (WB) and PCR as well as from 10 healthy individuals with no high-risk behaviour. DNA extracted from both plasma and peripheral blood mononuclear cells was amplified by PCR, using multiple primer pairs for distinct regions of the HIV-1 genome. Specificity of the PCR product was demonstrated by hybridization to an oligonucleotide probe. RESULTS All the index cases which were seropositive by ELISA and WB were also positive by PCR of plasma extracted DNA. Eight of the spouses were seronegative. Of these seven were positive by PCR--one spouse was negative by ELISA but showed a p55 band on WB and was positive by PCR. One spouse was negative by serology and PCR. The spouse belonging to the lone concordant couple was positive by serology and PCR. Except for one index case, PCR signals were obtainable only from DNA extracted from plasma but not from the DNA extracted from peripheral blood mononuclear cells. The control samples were negative by serology and PCR. CONCLUSIONS It is possible to detect HIV-1 infection by PCR using DNA extracted from plasma even when the individuals are negative by ELISA and WB. It can help in the early counselling of HIV infected persons and their spouses.