An outbreak of pertussis in primary school-children in the St David's area of Pembrokeshire provided the opportunity to estimate pertussis vaccine efficacy. The estimate of efficacy was 88% when notified cases were used, but this fell to 68% when all children with bouts of coughing for two or more weeks were included. Notified cases were significantly less likely to have been vaccinated than other cases with similar symptoms. Therefore vaccine efficacy estimates based upon notified cases are likely to be biased. However, even the lower estimates suggest that pertussis immunisation is highly desirable and efforts to improve coverage should be increased.