This paper reports an investigation into the associations between the number of children in the family and the vaccination uptake performance of those children. Using data from child health computer system the pertussis vaccination status was studied in 3,694 children aged 12 to 14 months. The findings show a strong association between vaccination uptake and the number of children in the family. The uptake rates reduced steadily from 86% for families with only one child to 58% for families with five children or more. Children from larger families were not only less likely to complete their full course of pertussis vaccination, but were also vaccinated later. Logistic regression analysis, which allowed for other associated factors, estimated that one 'child unit' increase in family size decreases the odds of being vaccinated by a factor of 1.7 in rural and suburban families and 1.3 in inner-city families respectively. Health professionals involved with the immunisation service should use child health computer data to target their problematic populations more effectively to achieve high vaccination uptake rates.