STUDY OBJECTIVE: To assess the validity and accuracy of children's medical utilisation estimates from a health interview and diary and the possible consequences for morbidity estimates. The influence of recall bias and respondent characteristics on the reporting levels was also investigated. DESIGN: Validity study, with the medical record of the general practitioner (GP) as gold standard. In a health interview and three week diary estimates of medical utilisation of children were asked and compared with a GP's medical record. SETTING: General community and primary care centre in the Netherlands. PARTICIPANTS: Parents of 1,805 children and 161 GPs. MAIN RESULTS: The sensitivity of the interview (0.84) is higher than the diary (0.72), while specificity and kappa are higher in the diary (0.96; 0.64) than in the interview (0.91; 0.5-8). Recall bias, expressed as telescoping and heaping, is present in the interview data. Prevalence estimates of all morbidity are much higher in the interview, except for skin problems. Compared with a parental diary more consultations are reported exclusively by the GP for children from ethnic minorities (OR 1.6), jobless (OR 2.3), and less educated mothers (OR 2.6). CONCLUSIONS: Estimates of medical utilisation rates of children are critically influenced by the method of data collection used. Interviews are prone to introduce recall bias, while diaries should only be used in populations with an adequate level of literacy. It is recommended that medical records are used, as they produce most consistent estimates.