Epidemiological evaluations of the prevalence of asthma are usually based on written questionnaires (WQs) in combination with validation by clinical investigation. In the present investigation, we compared parental assessment of asthma among their preschool children in response to a WQ with the corresponding medical records in the same region. An International Study of Asthma and Allergies in Childhood (ISAAC)-based WQ was answered by 75% of the parents of 6295 children aged 1-6 yr. Clinically diagnosed asthma, recorded in connection with admissions to the hospital or a visit to any of the outpatient clinics in the same region, were analysed in parallel. Finally, a complementary WQ was sent to the parents of children identified as asthmatic by either or both of this approaches. In response to the WQ 5.9% were claimed to suffer from asthma diagnosed by a doctor. According to the medical records, the prevalence of clinically diagnosed asthma was 4.9%. The estimated prevalence among children requiring treatment for their asthma was 4.4%. The sensitivity of the WQ was 77%, the specificity 97.5%. In the 1-2 yr age group the sensitivity was only 22%. This WQ was able to identify 54% of the children with a medical record of asthma. Forty percent of the children claimed by their parents to be asthmatic had no medical record of asthma. An ISAAC-based parentally completed WQ provided an acceptable estimation of the prevalence of asthma in children 2-6 yr of age, although only half of the individual patients identified in this manner are the same as those identified clinically.