During the West Nile virus outbreak in Israel in 2000, a woman with chronic lymphocytic leukemia who was comatose as a result of West Nile virus encephalitis recovered after treatment with intravenous immunoglobulin (IVIG) [1]. Antibody titers against West Nile virus were 1:1600 in Israeli IVIG; in contrast, North American IVIG preparations had no detectable West Nile virus antibody [1, 2]. A second patient, a lung transplant recipient, also recovered from West Nile virus encephalitis after treatment with Israeli IVIG [3]. Six other subsequently treated patients have had variable outcomes: 2 improved, 2 had no improvement, and 2 eventually died [4] (C. Isada, oral communication; R. Babecoff, written communication). These anecdotal reports, although inconclusive, have stimulated interest in the use of passive immunization for treating severe West Nile virus disease.