An outbreak of gastroenteritis lasting for one week in August 1980 affected approximately 1,500 persons in a community in northern Georgia. Investigation included a telephone survey of the community, a survey of textile plant employees and junior high and high school students and staff, and a neighborhood door-to-door survey. An association between gastrointestinal illness and consumption of drinking water was shown for community residents, students, and school staff. Attack rates (0-68%) determined in 10 neighborhoods increased significantly (P less than 0.001) with proximity to a textile plant, the site of one of two known cross-connections between an industrial water system (which contained fecal coliform bacteria) and the community water system. A fourfold rise in titer of antibody to Norwalk virus was found in 12 of 19 serum pairs from patients. Norwalk virus illness associated with drinking water from a large municipal water system has not been documented previously. Norwalk virus may be an important cause of waterborne morbidity in the United States.