Humans infected with West Nile virus (WNV) may clinically present with symptoms that are suggestive of neurological infection. Nearly all treatments of WNV disease have been effective in animal models only if administered before or soon after viral challenge. Here, we evaluated whether a potent neutralizing anti-WNV humanized monoclonal antibody (MAb), hE16, could improve the course of disease in a hamster model when administered after the virus had infected neurons in the brain. Five days after viral injection, WNV was detected in the brains of hamsters by cytopathic assay, quantitative reverse-transcription polymerase chain reaction, and immunohistochemical staining of WNV envelope in neurons. Notably, 80%-90% of the hamsters treated 5 days after viral injection by intraperitoneal injection with hE16 survived the disease, compared with 37% of the placebo-treated hamsters (P ≤ .001). The hamsters that received hE16 directly in the brain also exhibited markedly improved survival rates, compared with those in the placebo-treated hamsters. In prospective experiments, hamsters with high levels of infectious WNV in their cerebrospinal fluid were also protected by hE16 when administered 5 days after viral injection. These experiments suggest that humanized MAbs with potent neutralizing activity are a possible treatment for human patients after WNV has infected neurons in the central nervous system.