There is still debate on the effect of dietetic factors on the prevalence or severity of asthma, as well as the interaction with other factors. We have analysed the prevalence and severity of asthma according to adherence to the Mediterranean diet (MD), its association with obesity and family life style. We performed a cross-sectional study on 14,700 children and adolescents in six health areas in our region. We used the International Study of Asthma and Allergies in Childhood questionnaire to collect asthma symptoms, dietary habits, anthropometric parameters, parental smoking habits and maternal education level. The diet pattern was analysed using a Mediterranean Diet Score. In the logistic regression we analysed the influence of adhering to the MD on the prevalence and severity of asthma, adjusted for the other parameters included. Greater adherence to the MD is associated with a higher risk of severe asthma (odds ratios = 2.26, 95% CI: 1.21-4.22) in girls of 6-7 yr. There was no significant relationship for the other asthma categories in the population studied. The results of our study do not support a protective effect of the MD on the prevalence or severity of asthma.