From 1986 to 1988, 24 states and Puerto Rico reported 50 outbreaks of illness due to water that people intended to drink, affecting 25,846 persons. The protozoal parasite Giardia lamblia was the agent most commonly implicated in outbreaks, as it has been for the last 10 years; many of the outbreaks were associated with ingestion of chlorinated but unfiltered surface water. Shigella sonnei was the most commonly implicated bacterial pathogen; in outbreaks caused by the pathogen, water supplies were found to be contaminated with human waste. Cryptosporidium contamination of a chlorinated, filtered public water supply caused the largest outbreak during this period, affecting an estimated 13,000 persons. Although the total number of reported water-related outbreaks has been declining in recent years, the few large outbreaks due to Cryptosporidium, Norwalk-like agent, Shigella sonnei, and Giardia lamblia caused more cases of illness in 1987 than have been reported to the Water-Related Disease Outbreak Surveillance System for any other year since CDC and the Environmental Protection Agency began tabulating these data in 1971.