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Background: On May 29, 2011, an outbreak of suspected hepatitis A was reported in a primary school in Anhui Province, China.We investigated this outbreak to identify the cause, mode of transmission and recommend control measure.Methods: We defined a suspected case as onset of jaundice or anorexia plus elevated alanine aminotransferase in any person of this school since April 1, 2011 and a confirmed case as a suspected case plus HAV-IgM(+).We selected 18 case-students and 22 control-students (HAV-IgM-) frequency matched by class to compare their food, water exposure and personal hygiene habits.Results: We identified 28 cases (attack rate: 4.0%) in students of this school including 26 confirmed cases and 2 suspected cases.The onset of illness was from May 7 to June 8, 2011.The attack rate in students in grades 1-3 was 8.3%, compared to 2.0% in grades 4-6 (risk ratio 4.2; 95% CI =1.6-11).There was no cafeteria in this school, and students usually bought snack food from vendors around school.School water was from a well.In the case-control study, 67% of cases and 18% of control drank unboiled water from school well (OR=9.0, 95%CI=1.7-51).Logistic regression for hand-washing score of after toilet use and before meals (every time=5, always=4, sometime=2, never=0) in cases and controls showed good personal hand-washing practice was protective factor (1 increased score, OR=0.81, 95%CI=0.68-0.98).Multivariable logistic regression showed that drinking unboiled water was independently associated with illness (OR=15, 95%CI=2.7-88), but not hand washing (OR=5.7, 95%CI=0.87-37).The school toilet was 1 meter from the well.Conclusions: This hepatitis A outbreak was mainly caused by drinking unboiled well water.We recommended change the well location and disinfecting the well water.