The physical development of 14 patients with spontaneous pneumothorax was studied. At the time of operation, the mean age of these 14 patients was 21.2 years, height 172.5 cm, weight 54.1 kg and Rohrer's index was 104.6, indicating marked ectomorphism. Changes in the height and body weight of these 14 patients were compared with the standard curves estimated from the national averages. The height of these patients was greater than the national average already at the age of 6 years, showed a marked increase at 11-14 years, but the increase slowed down from age 15 on. Their body weight was less than the national average at the ages of 11 and thereafter, and its increase was remarkably slower at the age 15 and beyond. Consequently, Rohrer's index was particularly low at the ages of 11-15 years. These results suggest that the ectomorphism of patients with spontaneous pneumothorax is already apparent in early childhood and is enhanced by the delay in the increase in weight as compared with height in the period of rapid physical development. The rapid increase in the vertical dimension of the thoracic cavity relative to the increase in its horizontal dimension in this period is considered to affect the intrathoracic pressure in the apical region and may promote cyst formation.