In 1974, 28 outbreaks of waterborne disease affected 8413 persons in the USA. Giardia lamblia was the cause of illness in 7 outbreaks. Chemicals caused 5 outbreaks; they were fluoride, chromate, phenol, furadan insecticide and cutting oil. Other outbreaks involved Shigella sonnei and Yersinia entercolitica. In most outbreaks, untreated or inadequately treated water was responsible.