First certain measurements were made from the radiographs of normal children, namely the transverse diameter of the heart, the width of the lower thorax, and the lung length, and from tables prepared from these it was possible to study heart size and lung growth by taking similar measurements in the radiographs of a group of children with cystic fibrosis or asthma who had yearly radiographs. Secondly the types of radiological abnormality seen in each of the two diseases were defined and codified. Finally the long term radiological changes were studied in relation to deterioration in the cystic fibrosis group, and to radiological improvement following therapy in a small group of severe asthmatic children who also had abnormalities in the initial radiograph.