West Nile virus infection has been spreading westward across the continental United States since 1999. Although it often presents as a mild, self-limiting viral illness, it can result in a devastating meningoencephalitis in some patient populations, particularly the elderly. We report in this article on two immunosuppressed transplant patients who developed a severe meningoencephalitis caused by mosquito-borne West Nile virus infection. Suggestions for the prevention, diagnosis, and treatment of West Nile virus infection in this patient population are described.