A total of 1648 waterborne outbreaks and 446,377 cases of waterborne disease have been reported in the United States since 1920. During the most recent period, 1981-1988, 248 waterborne outbreaks occurred in community (45%) and noncommunity (34%) water systems and from the ingestion of contaminated water from individual (11%) and recreational (10%) water sources. The average annual number of outbreaks reported in the period are only slightly less than reported during the previous period, 1971-1980, and is comparable to that reported during 1931-1950. Although, several large outbreaks have recently been reported, most waterborne outbreaks since 1971 have occurred in small community and noncommunity water systems. The average number of cases of waterborne disease per outbreak during 1971-1988 is less than reported during 1920-1940 but more than reported during 1941-1960.