Objective: To describe the effectiveness of containment of novel influenza A(H1N1) infection at a summer camp. Design: Targeted use of oseltamivir phosphate by individuals in close contact with influenza-confirmed cases. Setting: Boys' camp in Alabama in July 2009. Participants: A total of 171 campers, 48 camp counselors, and 27 camp staff. Interventions: Campers with confirmed influenza received oseltamivir and were immediately isolated and sent home. All boys and counselors in the infected child's adjoining cabins received prophylactic oseltamivir for 10 days, including 8 campers at higher risk for influenza infection (eg, those with asthma, seizure disorder, or diabetes). Alcohol-based hand sanitizer was provided at each of the daily activities, in the boys' cabins, and in the dining hall, and counselors were educated by the medical staff on the spread of influenza and its prevention through good hand hygiene. All cabins, bathrooms, and community sports equipment were sprayed or wiped down with disinfectant each day. Main Outcome Measure: Virologic confirmation of influenza. Results: Three of the 171 campers tested positive for influenza A during the course of the 2-week fourth session, for an attack rate of 1.8%. The probability of observing 3 or fewer infected campers if the attack rate was 12% is less than 1 in 10 000 000 (P<.0000001). An exact 95% confidence interval based on 3 events among 171 individuals estimates the attack rate to be between 0.3% and 5.0%. While 31% to 57% of campers, counselors, or staff experienced nausea with the treatment, this did not result in discontinuation of therapy. No campers tested positive for influenza A after returning home at the end of the camp session. Conclusion: In conjunction with comprehensive hand sanitization and surface decontamination, a targeted approach to antiviral prophylaxis contained the spread of influenza in a summer camp setting. ©2010 American Medical Association. All rights reserved.