AIMS:To estimate the relationship betweenidiopathicthrombocytopaenic purpura(ITP) and the measles, mumps and rubella (MMR) vaccination in children; calculating the relative risk estimate for ITP with in 6 weeks afterMMRvaccination and the attributable risk of ITP within 6 weeks afterMMRvaccination.METHODS:Using the General Practice Research Database we identified children with a first-time diagnosis of ITP from a base population of children aged less than 6 years between January 1988 and December 1999. After describing the characteristics of all the children identified with ITP, we focused on cases aged 13-24 months to perform a population-based, case-control analysis to estimate the relative risk of developing ITP within 6 weeks afterMMRvaccination. We also calculated the risk of ITP attributable to theMMRvaccination.RESULTS:Sixty-three children with a first time diagnosis of ITP were identified; 23 cases were between 13 and 24 months old. The relative risk estimate for ITP within 6 weeks afterMMRvaccination, compared to the combined group of unvaccinated children and children vaccinated withMMRmore than 26 weeks previously was 6.3 (95% CI 1.3-30.1). The attributable risk of developing ITP within 6 weeks afterMMRvaccination was estimated to be 1 in 25,000 vaccinations (95% confidence interval 21,300, 89,400).CONCLUSION:This study confirms the increased risk of ITP within 6 weeks afterMMRvaccination. However, the attributable risk of ITP within 6 weeks afterMMRvaccination is low.