The number of proviral copies in the cerebrospinal fluid of 13 patients in various stages of HIV-1 infection was determined using the polymerase chain reaction. HIV-1 proviral DNA was detected at a median value of 1 provirus per 300 cells with a range from 1 per 20 to 1 per 2,400 cells and a mean value of 1 provirus per 570 cells. Interestingly, this indicates a higher proportion of infected cells in cerebrospinal fluid than is reported to be the case for blood mononuclear cells. However, a clear correlation of these findings to the CDC stage or to neurologic complications was not obvious.