To study lung development after repair of congenital diaphragmatic hernia 20 patients operated on between 1960 and 1976 were followed-up at 6-22 years of age. All had had an uneventful postoperative course, had no concomitant disease and were subjectively well with an ordinary physiological performance. The investigation showed a mean increase of residual volume by one third of predicted normal values. Fractional perfusion and ventilation were reduced by 20% on the hernia side compared with the contralateral side. Functional impairment owing to pulmonary hypoplasia was ruled out by the existence of a normal working capacity and maintenance of a normal alveolar gas exchange during exercise on a high work load.