During the summer of 2000, cases of West Nile virus (WNV)-associated encephalitis were identified in horses in the Montpellier region. Very quickly, surveillance was established to monitor potential clinical cases of encephalitis and meningitis in humans and horses occurring in the area. This surveillance was supplemented by a serological survey of all horses within a 10-km radius of equine clinical outbreaks and of 400 wild and domestic birds in the area. This article presents, section by section, the methods and results of the surveillance program implemented during the summer of 2001. The strengths and limitations of such a program are then discussed. (adapted from the text)