We present data on the distribution of human immunodeficiency virus (HIV-1) proviral DNA in different subsets of peripheral blood mononuclear cells (PBMCs) over an observation period of eight months. Eleven patients with well documented HIV-1 infection were studied. The PBMCs were obtained at two intervals and purified by fluorescence-activated cell sorting (FACS) after staining with FITC-labelled monoclonal antibodies. Varying numbers of FACS-sorted CD4+ cells, CD8+ cells and peripheral monocytes were assayed for HIV-1 proviral DNA (env and gag region) by PCR. Samples from patients at CDC stages II or III had to contain 10(3)-10(4) cells in order to allow detection of proviral HIV-1 DNA. At CDC stage IV, however, HIV-1 DNA was detected in as few as 100 CD4+ T-lymphocytes. In contrast, in peripheral monocytes HIV-1 DNA was not regularly found. CD8+ cells did not harbor detectable amounts of proviral DNA. During an observation period of eight months, the rate of infected CD4+ T-lymphocytes increased significantly in three patients while staying constant in the remaining eight patients. This increase of the infection rate was paralleled by clinical progression in one patient and by a decrease of the absolute number of CD4+ cells in another patient. The percentage of CD4+ cells harboring the viral genome increases in the course of the disease. These results may help to explain the decrease in CD4+ T-lymphocyte counts during HIV-1 infection.