Radiotherapy is known to have acute and long term deleterious effects on lung tissue. However, pulmonary irradiation is an established treatment in advanced childhood tumours with pulmonary metastases not responsive to chemotherapy. In this study eight patients with Wilms' tumours and lung metastases treated with whole lung irradiation (1200-1837 cGy) and chemotherapy were reassessed clinically, radiologically and with lung function tests 6-26 years after radiotherapy. One patient was breathless after mild exertion, four after strenuous exercise and three were asymptomatic. Clinically all had small chests and four of five females had underdeveloped breasts. A chest radiograph showed clear lung fields in all cases. Lung volumes, especially total lung capacity (TLC) and vital capacity (VC), were decreased when compared with predicted values for age and height. However, gas transfer per unit lung volume (KCO) was normal. This study suggests that pulmonary irradiation in childhood results primarily in underdevelopment of the thorax and that diffuse interstitial lung fibrosis is not a significant feature at this dose level.