This study investigated household transmission data for influenza (H1N1-2009) in Japan in order to quantify the age-specific risk of infection and estimate the impact of antiviral treatment on the risk of household transmission. Among a total of 1547 households, involving 4609 household contacts, the secondary attack ratio (SAR) was estimated to be 11.4%. School children aged 5 - 18 years dominated the index cases. Age-specific infectiousness and susceptibility were highest among 0 - 4-year olds, with SAR estimated at 19.4% and 29.6%, respectively. Zanamivir treatment within 24 and 24 - 48 h of illness onset in index cases, respectively, reduced the risk of household transmission to 0.57 (95% CI 0.44, 0.73) and 0.58 (95% CI 0.38, 0.86) times that among those receiving the same treatment at > 48 h and those not receiving treatment. The preventive performance of antiviral treatment and prophylaxis should be further examined in randomized controlled trials.