Background. The interval between bronchodilator administration and post-bronchodilator lung function testing is critical for accurate interpretation of the bronchodilator response. The time course of this response in children is not well documented. We aimed to document the time taken to achieve maximal lung function following salbutamol inhalation. Methods. Eighteen asthmatic children between 7 and 18 years of age with a history of bronchodilator responsiveness were recruited. Spirometry was performed before and at 0, 10, 15, 20, 40, 60, and 90 minutes after salbutamol inhalation 600 μ g (Ventolin; GlaxoSmithKline) via a spacer (Volumatic; GlaxoSmithKline). Results. Spirometric indices significantly increased after salbutamol inhalation (p <0.001). The group median time to maximal response in forced expiratory volume in 1 second (FEV1) was 17.5 (10–60: 10th–90th centiles) minutes after salbutamol. The magnitude of group response in FEV1 was significantly higher at 15 and 20 minutes than at 0 and 10 minutes post-salbutamol inhalation (repeat measures analysis of variance [ANOVA] on ranks; p <0.05). Conclusion. We conclude that a minimal interval of 20 minutes, before re-testing spirometry, is required to document the maximal response to bronchodilators in the majority of asthmatic children.