Peripheral blood of 57 patients with antibodies to human immunodeficiency virus 1 (HIV-1) and of five HIV-1 seronegative subjects at risk for HIV-1 infection were analysed by polymerase chain reaction (PCR) and virus isolation. The virus was recovered from peripheral blood cells in 89% and from plasma in 75% of the HIV-1 seropositive cases. In contrast, proviral HIV-1 DNA was detected in all HIV-1 seropositive patients by dot blot hybridization of the amplified fragments. The intensities of the dot blot reactions were less pronounced in asymptomatic HIV-1 seropositive individuals than in patients with acquired immunodeficiency syndrome (AIDS) or AIDS-related complex (ARC), suggesting an increase in proviral DNA with advancing disease. Three of five seronegative patients with signs or symptoms suggesting HIV-1 infection, but none of the controls, were positive for HIV-1 DNA by one or two primer pairs. These results show a high sensitivity of the PCR for detecting HIV-1 DNA in patients of all stages of HIV-1 infection. Proviral DNA can also be detected in some individuals without detectable antibodies to the virus. The virus load in peripheral blood, as determined by virus cultivation and PCR, seems to increase with progression of the infection.