Pertussis in young children is severe and relatively prevalent in vaccinated populations. We estimated the impact of pre-school booster vaccination of 4-6-year-old children on pertussis in 0-1-year-old children. We conducted a population-based historical cohort study of all children born in Denmark, 1977-2001 (N = 1,536,717) using information on place of residence to identify household members and vaccination history from nationwide registers. We estimated rate ratios (RRs) of pertussis hospitalisation among children in the cohort according to number, age, and vaccination status of their household members. This enabled, through population attributable risks, the estimation of the preventable proportion of hospitalisations among 0-1-year-old children according to age at booster vaccination (4-6 years), booster uptake, and the efficacy of the booster against transmission. The preventable proportion of pertussis hospitalisations among 0-1-year-old children ranged from 7% to 33% (most realistic scenario = 18%), varying according to age at booster vaccination, uptake, and efficacy of booster against transmission. This relatively limited impact of a pre-school booster was partly a consequence of the actual number of 0-1-year-old children living with children of pre-school age or older and partly the result of significant exposure from children younger than pre-school age in the household. According to our model the effectiveness of pre-school booster vaccination as an intervention to prevent pertussis hospitalisation of 0-1-year-old children is modest.