Outbreaks of acute gastroenteritis are a public health problem. Norovirus is known as the most common cause (50%). In Chile, immediate notification allows surveillance of these events. We describe an acute gastroenteritis outbreak that occurred in Antofagasta region, between March and April 2010. An observational study was conducted to perform the outbreak investigation. Local residents who met case definition were included. Stool samples, epidemiological surveys and environmental samples were requested. The outbreak began approximately on March 8, 2010 and lasted until April 28 with 31,036 reported cases (rate 54 per 1000 inhabitants). The most affected age group was between 25 and 44 years, and diarrhea was the main symptom (97% of cases). We determined the presence of norovirus genogroup II in clinical and environmental samples. This outbreak was caused by consumption of raw vegetables from La Chimba, which were watered and contaminated with treated sewage containing low concentration of free residual chlorine. Subsequently, the outbreak spread from person to person in a poor sanitary environment.