A less expensive approach-and the one taken in the Realtime Outbreak and Disease Surveillance (RODS) system-is detection based on data collected routinely for other purposes. Examples of such data include absenteeism data, sales of over-the-counter (OTC) health care products, and chief complaints from EDs. 9 The expenses of manual data collection are avoided; however, the data obtained typically are noisy approximations of what could be obtained by direct interviewing of the patient (in the case of individual level data). Both approaches may play complementary roles with current methods of public health surveillance 10-12 by assisting the physician and public health official with a continuously updated picture of the ''health status'' of a population. 13, 14 A focus of our research has been syndromic surveillance from free-text chief complaints routinely collected by triage nurses in EDs and acute care clinics during patient registration. We have deployed this type of surveillance at the 2002 Winter Olympics and in the States of Pennsylvania and Utah. We described a previous version of the RODS system, 12 but the system has undergone considerable subsequent development both architecturally and functionally. This report provides a detailed description of the current version of RODS, an example of a computer-based public health surveillance system that adheres to the National Electronic Disease Surveillance System (NEDSS) specifications of the Centers for Disease Control and Prevention (CDC). 15, 16 Background