We investigated the association between introduction of smoke-free legislation in the UK (March 2006 for Scotland, April 2007 for Wales and Northern Ireland, and July 2007 for England) and the incidence of respiratory diseases among children. We extracted monthly counts of new diagnoses of wheezing/asthma and RTIs among children aged 0-12 years from all general practices in the Clinical Practice Research Datalink during 1997-2012. Interrupted time series analyses were performed using generalised additive mixed models, adjusting for underlying incidence trends, population size changes, seasonal factors, and pandemic influenza, as appropriate. 366,642 new wheezing/asthma diagnoses and 4,324,789 RTIs were observed over 9,536,003 patient-years. There was no statistically significant change in the incidence of wheezing/asthma after introduction of smoke-free legislation in England (incidence rate ratio (IRR) 0.94, 95% CI 0.81-1.09) or any other UK country (Scotland: IRR 0.99, 95% CI 0.83-1.19; Wales: IRR 1.09, 95% CI 0.89-1.35; Northern Ireland: IRR 0.96, 95% CI 0.76-1.22). Similarly no statistically significant changes in RTI incidence were demonstrated (England: IRR 0.95, 95% CI 0.86-1.06; Scotland: IRR 0.96, 95% CI 0.83-1.11; Wales: IRR 0.97, 95% CI 0.86-1.09; Northern Ireland: IRR 0.90, 95% CI 0.79-1.03). There were no demonstrable reductions in the incidence of paediatric wheezing/asthma or RTIs following introduction of smoke-free legislation in the UK.