In this 2005 study from the Los Angeles County Public Health Department Laboratory, Los Angeles, CA, cerebrospinal fluid specimens submitted for West Nile virus (WNV) serologic testing were also tested for enterovirus by real time reverse transcription-polymerase chain reaction and culture. Approximately 10% of the WNV- specimens were positive for enterovirus. Altered consciousness and muscle weakness were more common with WNV infection. Results provide compelling evidence that laboratory testing is a necessary adjunct to clinical diagnosis for accurate discrimination among the various possible etiologic agents of febrile and central nervous system disease.