In mid-June 2009, an outbreak of pandemic (H1N1) 2009 (pH1N1) infection occurred in a secondary school in Hong Kong. We carried out an epidemiological investigation to delineate the characteristics of the outbreak, gauge the extent of secondary household transmission, and assess the protective role of oseltamivir in household contacts. We interviewed pH1N1-confirmed cases using a standardized questionnaire. Sixty-five of 511 students in the school were affected. Of the 205 household contacts identified, 12 were confirmed as cases. All cases recovered. The estimated secondary household attack rate was 5·9% (95% CI 2·7-9·1). Household contacts aged <18 years were about 15 times more likely to be infected than older contacts. Household contacts who had received oseltamivir prophylaxis were less likely to acquire a secondary infection than those who had not (odds ratio=0). The estimated mean household serial interval of pH1N1 virus was 2·8 days (95% CI 2·1-3·4 days).