Background: The international study of asthma and allergies in childhood (ISAAC) is used to define the prevalence and severity of asthma in different regions. In this study we followed the performance of the ISAAC video and written questionnaires (VQ and WQ) to classify asthma in 13-14 yr-old schoolchildren. Methods: The present study was carried out on 3540 schoolchildren 13 to 14-yrs-old using the VQ and WQ. The students were also asked to answer a separate question if they had ever been clinically diagnosed as asthmatic. The Kappa index was used to find degree of agreement between VQ and WQ regarding asthma symptoms. The sensitivity, specificity and Youden’s index of both questionnaires were also determined. Results: The most frequent asthma symptom was wheeze after exercise in both VQ (15%) and WQ (23%). Correlations between positive responses to the corresponding questions in the VQ and WQ were significant (P<0.001). The sensitivity of both questionnaires to the question of asthma diagnosis by physician ranged between 0.16 and 0.68. Except for wheeze limiting speech and current wheeze, the WQ had higher value of Youden’s index (0.25 - 0.46). The specificity of questions in VQ for asthma diagnosis by physician was ranged between 0.84 and 0.99. Conclusion: Both WQ and VQ had low sensitivity, but WQ showed higher value of Youden’s index and the VQ exhibited good specificity. Therefore, one may consider the WQ for prevalence and VQ for relative risk studies.