The ventilatory function of 230 patients with pneumonectomy, performed at ages ranging from 2 to 40 years, has been followed for more than 30 years (mean 33 years). We have tried to analyze whether the available data gave information about the nature and the mechanisms adapting the remaining lung to the larger than normal pleural space and about the persistence of this adaptation in the longer term. There were 32 patients with a persistent, disturbed forced expiration (mean 50% of vital capacity). This group of patients was excluded from the study to answer the above questions, because the subdivisions of the total lung capacity in this group differed significantly from those in the group of 98 patients with a normal forced expiratory volume of 72% (mean) of vital capacity. The data of the 98 patients, who were subdivided into seven age groups at the time of pneumonectomy, permitted the following conclusions: In the youngest age group (0 to 5 years), the ventilatory capacity is hardly smaller than the predicted capacity for two lungs; this suggests that compensatory growth by way of hyperplasia might have been the most important adaptive mechanism in this group. In the age group 6 to 20 years, a significant difference is still found as compared to the group of patients operated on at an older age; this difference indicates that in this period compensatory growth, possibly mainly simple hypertrophy, still played an important but gradually decreasing role. The fact that the effect of the adaptational mechanisms could be observed more than 30 years after ablation of one lung, without loss in quality of function (i.e., forced expiratory volume constituting a normal percentage of the vital capacity), indicates that the adaptive mechanisms also compensate for the loss in lung tissue in the longer term. A striking finding was the stability of the tidal volume/functional residual capacity ratio, which, especially in the younger age groups, was very close to the predicted value for two lungs. This finding is in agreement with the fact that most persons with a healthy remaining lung lead a normal family and social life after pneumonectomy.