Cases of acute pneumothorax admitted in the period 1975-1978 were examined to determine the presence of changes in ventricular repolarization. A significant elongation of the QT segment was noted in 23/126 cases. The series did not include subjects with chronic heart and /or bronchopulmonary disease, or other clinical features that might explain such elongation. The ECG changes bore no relation to the site and extent of the pneumothorax. Agreement is expressed with the view that their explanation lies in the existence of a reflex mechanism acting on the autonomous nervous system.