Sixty-nine children with asthma and 86 with cystic fibrosis (CF) were assessed for lung function, respiratory muscle function and exercise tolerance. Thirty-seven with asthma and 41 with CF subsequently undertook an at home exercise program and the remainder were matched controls. There was no significant change in pulmonary function after 3 months of the exercise program. Compliance with the program decreased markedly towards the end of the 3 months and this lack of compliance may have contributed to the failure to show any significant training effect. A previously supervised program showed increased fitness and increased peak pressures in children with asthma following intensive exercise. There appears limited value in promoting unsupervised home exercise programs for children with CF and asthma. In those groups of children who will benefit with increased physical activity, supervised programs and more interesting play activities may need to be organized to obtain optimal benefit.