The data presented in this article relate to waterborne disease outbreaks associated with water obtained from public or individual water systems and intended for drinking or domestic purpose during the thirty-five-year period between 1946 and 1980. The occurrence, distribution, and trends of such outbreaks are discussed. Between 1946 and 1980, 672 waterborne disease outbreaks affecting more than 150,000 persons were reported. Outbreaks exhibit temporal distribution, with peak occurrences in June, July, and August. Although outbreaks in noncommunity systems outnumber those in community systems, the frequency of outbreaks is almost twice as great in community systems. Cases of illness occurring in community water systems far outnumber those in noncommunity systems. The deficiencies in water systems that caused and contributed to waterborne outbreaks during this period differed little from those reported for the previous 26 years (1920-1945). The glaring deficiencies were that disinfection was not in place where it was needed and not properly operated where it was in place. Includes 8 references, tables, figures.