The humoral immune response to HIV infection plays an important role in determining disease progression. Few and discordant results correlate changes in neutralizing antibody (NtAb) titer with antiretroviral treatment. The NtAb titer against autologous-HIV was evaluated in 33 patients treated with the protease inhibitor saquinavir (SQV, Invirase) and zidovudine (ZDV) alone or in combination. Ten out of 33 (30%) patients showed a significant increase (4-fold or greater) in NtAb titer from baseline in response to the initiation of therapy. A significant correlation (P = 0.007) was found between an increase in NtAb titer and treatment with SQV alone (5 subjects) or in combination (5 subjects). A significant decrease in NtAb titer was detected in 7 patients, 5 of whom were treated with ZDV alone. After one year of therapy a significant decrease in HIV-RNA copy number (> 0.5 log) with respect to baseline value was detected only in patients treated with SQV alone or in combination. Patients with increased NtAb titer showed a significantly reduced HIV-RNA copy number and increased CD4+ cell count at week 16 of treatment which were sustained up to week 52. These data suggest that treatment with SQV can improve neutralizing activity against autologous virus as well as bring about a significant and sustained reduction in viral load.