Serum specimens (n = 161) from 31 persons before and after human immunodeficiency virus type 1 (HIV-1) seroconversion were tested for anti-CD4 antibodies. These antibodies were detected by both ELISA and Western blot in 55% (17/31) of subjects when HIV-1 seroconversion was detected and in 26% (8/31) from sera obtained 6-24 months earlier. A decrease in CD4+ cell number was associated more with development of anti-CD4 antibodies or peak anti-CD4 antibody activity than with development of anti-HIV-1 antibodies. Quantitative DNA polymerase chain reaction assay of peripheral blood mononuclear cells from 7 seroconverters showed evidence of HIV-1 infection in 4 of 4 specimens obtained after HIV-1 seroconversion but was nonreactive for 12 of 12 specimens obtained before HIV-1 seroconversion, including 4 specimens positive for anti-CD4 antibodies by ELISA and Western blot. Therefore, anti-CD4 antibodies are frequently present in the sera of HIV-1-infected persons before and at the time HIV-1 seroconversion is detectable and are associated with a decline in CD4+ cell counts, but they are not a marker for HIV-1 infection in seronegative persons.