We wanted to establish whether the reduced risk of asthma in children with diabetes had a genetic basis or was in some way related to the diabetic phenotype. Control data for the frequency of symptoms associated with asthma and other atopic conditions are available from a large published survey of children aged 12-14 years, including 2324 children from the Oxford and East Anglia region. 3 is a population-based prospective study that has recruited 1627 families in the former Oxford Regional Health Authority area (population 2·6 million) since 1985. Our database identified 206 probands and 209 unaffected siblings aged 12-14 years within this cohort. Of these, 181 probands and 184 siblings agreed to take part, and 157 and 173, respectively, returned completed questionnaires. Data were therefore available for 76% of the eligible probands and 83% of the siblings. The International Study of Asthma and Allergies in Childhood (ISAAC) questionnaire of symptoms of asthma and allergy 4 was used in all groups. Only 27% of probands had ever experienced wheeze compared with 42% of the siblings and 51% of controls ( 2 for trend 34, p<0·001). Wheeze within the previous year was reported by 17% of the probands, 28% of the siblings, and 34% of the controls ( 2 for trend 21, p<0·001). The odds ratio for ever wheezing in children with type-1 diabetes compared with controls was 0·36 (0·25-0·52), and that for wheezing within 12 months was 0·39 (0·25-0·60, figure A). Symptoms related to severity, ie, more than four attacks of