Objective To study the epidemiological characteristics of an outbreak caused by norovirus infection in a school in Haidian district,Beijing.Methods Basic information of the school and data related to patients in the fields survey were collected and analyzed descriptively.Laboratory tests were performed to test the stool and anal swab specimens of both patients and cooks as well as the environmental specimens.Risk factors related to the incidence were analyzed through a case-control study.Results A total number of 119 patients were identified in the school.Clinical symptoms were mild,mainly involving vomiting (94.1％,112/119),abdominal pain (46.2％,55/119),but no need of hospitalization.The average age of the student patients was 6.38,with minimum and maximum between 5 and 11.Patients were found in 22 classes,but mainly in grade 1 and class 7 where 35 patients were found (30.17％).A total of 134 specimens of rectal swabs and stool were collected,with 7 positive for norovirus and 6 for sappovirus.Salmonella,Shigella,lapactic Escherichia coli and Vibrio parahaemolyticus were not found in on dinner sets,residual foods,bottled water or in drinking fountains.Index on water hygiene was unsatisfactory in classrooms or dormitories where more cases were found.Accommodation,north-facing-classrooms,abnormal water hygiene indexes were found related to the occurance of the disease (P＜0.05).Conclusions The outbreak was identified a gastroenteritis infection,caused by norovirus with symptoms as vomiting and abdominal pain.This event reached the reporting standards of public health emergencies-level Ⅳ.Discovery and isolation of the first case was not timely while transmission of the disease might be water-borne.Surveillance programs on symptoms,disinfection of vomit and stool in places like nurseries and schools should be strengthened to prevent the norovirus outbreak.