Urbanized areas often discharge large volumes of contaminated waste into coastal waters, which may pose a health risk to bathers at nearby beach areas. In this investigation the authors estimated the number of gastrointestinal and respiratory illness episodes associated with the microbial contamination of coastal waters among bathers at Southern California beaches from 2000 through 2004. Bathers at the 67 beaches along the 350-km coastline of Southern California were the study population in this investigation. The authors' estimates were derived from a simulation model, which utilized water quality, beach attendance, and bathing-rate data, along with the three concentration-response relationships that underlie US Environmental Protection Agency, World Health Organization, and European Union marine water-quality guidelines. Given the absence of a general surveillance program to monitor these illnesses in Southern California, simulation modeling provides an established method to derive health risk estimates, despite additional analytic uncertainty that may accompany modeling-based analyses. An estimated 689,000 to 4,003,000 gastrointestinal illness episodes and 693,000 respiratory illness episodes occurred each year. The majority of illnesses (57% to 80%) occurred during the summer season as a result of large seasonal increases in beach attendance and bathing rates. As 71% of gastroenteritis episodes were estimated to occur when the water quality was considered safe for bathing, California's marine water-contact standards may be inadequate to protect the health of bathers.