Thirty-two waterborne disease outbreaks were reported to the Centers for Disease Control (CDC) and the Environmental Protection Agency in 1981. The outbreaks occurred in 17 states and involved 4430 cases. This was only 64% of the number of outbreaks that were reported in 1980 and the total number of individuals involved in 1981 was less than one-quarter of that reported for the previous year. The causal agent was unknown in 44% of the 1981 outbreaks; Giardia lamblia was identified as the etiological agent in 28%, and chemical agents caused 16% of the outbreaks. The remaining 12% of the outbreaks were caused equally by Shigella, Campylobacter, V. cholera 01 and rotavirus. Fifty-six percent of the outbreaks occurred in private or noncommunity water systems and the remainder were observed in community public supply systems. Most of the outbreaks (75%) were caused by the use of untreated or inadequately treated water.