The prevalence of allergic diseases in childhood has increased significantly over the last decades. This increase seems to be closely associated with the way of life of western societies. The high prevalence differences on different regions may be due to linguistic and cultural reasons and not to real variations in prevalence. This is the reason why several authors felt the need to perform an objective validation of their versions. Our working group has published the results of the Phase I validation and now is publishing the Phase III validation in order to guarantee the reliability of this phase results. The study sample is formed by 366 children aged 3 to 17 years. The following steps were followed in this study: I. Assessment of the "Criterion validity" of the Spanish ISAAC-Bronchial Asthma questionnaire, evaluating the sensitivity, specificity, relative value, and positive and negative predictive values. 2. Determine the questionnaire reliability, analysing its "Inner consistency". 3. Statistical comparison between our ISAAC-Bronchial Asthma results and the ones obtained by other groups (external concordance and consistency), in order to prove the previously evaluated reliability. 4. Comparison between the ISAAC-Bronchial asthma questionnaire diagnostic ability and the standard diagnostic criteria universally used in clinical praxis. We could confirm that there is a high and very significant concordance between the questions aimed to detect children with asthma. In this sense, it is especially useful the question about "ever had wheezing" because of its high sensitivity (93.3%) and specificity (89.9%), that make it able to be used as initial screening test in a general population, and that has shown a high concordance percentage with the questions "ever had asthma" (98%), "wheezing with exercise" (75%), and "cough at night"(80%). The questions that give more information about the evolution and control of the asthmatic disease are "wheezing in the last 12 months", "number of attacks in the last 12 months", "wakening at night", "wheezing with exercise" and "dry cough at night in the last 12 months". The questions more related to asthma severity were "number of attacks in the last 12 months", "wakening at night", "stop speaking in order to breath", and "wheezing with exercise". We conclude that ISAAC-Asthma questionnaire Phase III is a useful tool for the assessment of childhood asthma due to its criterion validity, inner consistency and external concordance.