A cluster of acute gastroenteritis among nursing students was noticed on 13th September 2005. Between 13th and 17th September 2005, a retrospective cohort study was then conducted to identify the most likely cause of gastroenteritis at a nursing college in Bangkok, Thailand. Self-administered questionnaires, interviews, environmental investigations, and rectal swabs from all participants were carried out. In the investigation, 98.9% female nursing students were investigated and had completed the questionnaire, 49.4% of the participants were diagnosed to have acute gastroenteritis. The predominant symptoms were watery diarrhoea (90.8%) and abdominal cramps (71.3%). Of 28.9% of rectal swab isolates were identified as Vibrio parahaemolyticus O4:K55 (40.4%), Salmonella spp. (19.2%), Vibrio cholerae non O1/non O139/non O141 (11.5%), Aeromonas trota (3.9%), Vibrio alginolyticus (1.9%) and other co-infections (23.1%). The tdh gene was identified from all V. parahaemolyticus using multiplex PCR. The implicated food risk factor for gastroenteritis was boiled egg (adjusted prevalence rate ratio; PR=1.9, 95% CI, 1.04 - 3.79). However the bitter melon soup was not significantly associated for gastroenteritis (adjusted PR=1.3, 95% CI, 0.98 - 1.82). The population attributable fraction analysis indicated that boiled eggs item was an implicated food risk for this outbreak (PAF=45.8%). Vibrio parahaemolyticus O4:K55 was identified as a major causative agent for gastroenteritis in which the contaminated boiled eggs was a vehicle in this outbreak. Cross-contamination control should be emphasized in food operation plans among