Whilst the lung is a major target for opportunistic microorganisms in the later stages of AIDS the role of HIV-1 infection of the lung in the disease process remains controversial. To clarify this question we have used limited cell dilution PCR for the detection of HIV-1 proviral DNA and have also cultured the virus. Limited cell dilution PCR for the detection of HIV-1 DNA from PBMC and alveolar cells indicated that more cells were infected with HIV-1 in matched PBMC samples from 7 out of 9 individuals. Closer examination of purified alveolar lymphocytes and alveolar macrophages showed that higher numbers of alveolar lymphocytes (10 out of 14) had detectable HIV-1 proviral DNA. Interestingly HIV-1 could be isolated in culture from 8 out of 14 alveolar lymphocyte samples and 9 out of 14 alveolar macrophage samples. In this respect the removal of CD8+ alveolar lymphocytes from the cultures greatly facilitated the replication of HIV-1 in alveolar macrophages. Our conclusion is that the replication of HIV-1 is extremely efficient in alveolar macrophages when CD8+ lymphocytes are not present and therefore AM are an important source of HIV-1 in AIDS.