Longitudinal changes in neutralizing antibody responses against autologous human immunodeficiency virus (HIV) type 1 were investigated in 19 chronically infected patients who were undergoing highly active antiretroviral therapy (HAART). Reconstitution of or increase in neutralization activity was observed in 4 of 19 patients during HAART, but neutralization activity was more or less unchanged in most patients. Three of 4 patients with increased neutralization activity had low-level viral rebound ("blips") during HAART. No correlation was found between neutralization activity and HIV-specific CD4+ T cell frequencies. There was a correlation between neutralization activity and CD4+ T cell counts. The reconstituted antibody represented limited cross-reactivity, compared with that of preexisting antibody. Binding activity to monomeric gp120, V2, and V3 peptides was reduced. Both prolonged virus suppression, for CD4+ T cell recovery, and blips, for stimulation of the immune system in vivo, may be required for development of neutralizing antibody in vivo.