OBJECTIVES: This report describes the results of an investigation on an outbreak of novel influenza A (H1N1) in an English language Institute in Seoul, Korea in May 2009. METHODS: In this outbreak, novel influenza A (H1N1) was confirmed in 22 of 91 trainees, trainers and staff members. The trainees and 2 staff members were isolated in an assigned facility and the rest were isolated in their homes after we discovered the first patient with novel influenza A (H1N1). After the isolation, the people in the assigned facility were educated to use N95 respirators and they received oseltamivir for prophylaxis. RESULTS: The initial findings in this study suggest that the symptoms were mild and similar to those of seasonal influenza. The classmates and roommates of the infected patients were more likely to get infected with novel influenza A (H1N1) than the trainees who were not classmates or roommates of the patients (OR: 3.19, 95% CI=0.91 - 11.11 for classmates and OR: 40.0, 95% CI=7.4-215.7 for roommates). CONCLUSIONS: The public health response seems successful in terms of preventing the spread of this virus into the local community.