Diseases of the central nervous system (CNS) such as meningitis or encephalitis may represent events of public health interest due to emerging infections and/or NIH/CDC Category B priority pathogens. Apart from influencing treatment and management of the index case, some diagnoses such as meningococcal meningitis warrant an immediate public health response. Others such as West Nile Virus may require public education and vector control. Thus early detection of CNS syndromes is of benefit to patients, providers and public health. While computer-based surveillance methods have been used with success in the early detection of respiratory syndromes, there is little data on their use in CNS syndromes. This study analyzed the validity of a hospital emergency department computer-based surveillance system in the early detection of meningitis and encephalitis and determined the test characteristics of selected computer-based rules.