The neutralizing Ab response after primary HIV-1 infection is delayed relative to the virus-specific CD8(+) T cell response and the initial decline in plasma viremia. Because nearly all HIV-1 infections result in AIDS, it would be instructive to study cases where neutralizing Ab production commenced sooner. This was done in subject AC10, an individual treated during early infection and in whom a rapid autologous neutralizing Ab response was detected after therapy cessation as rebound viremia declined and remained below 1000 RNA copies/ml of blood for over 2.5 years. This subject's Abs were capable of reducing the infectivity of his rebound virus by >4 logs in vitro at a time when rebound viremia was down-regulated and virus-specific CD8(+) T cells were minimal, suggesting that neutralizing Abs played an important role in the early control of viremia. The rebound virus did not exhibit an unusual phenotype that might explain its high sensitivity to neutralization by autologous sera. Neutralization escape occurred within 75 days and was proceeded by neutralizing Ab production to the escape variant and subsequent escape. Notably, escape was not associated with a significant rise in plasma viremia, perhaps due to increasing CD8(+) T cell responses. Sequence analysis of gp160 revealed a growing number of mutations over time, suggesting ongoing viral evolution in the face of potent antiviral immune responses. We postulate that an early effective neutralizing Ab response can provide long-term clinical benefits despite neutralization escape.