In a prospective study, 85 patients underwent simple observation for minor injuries of the pleural cavity as a result of penetrating injury of the chest. Seven patients (8 per cent) required delayed drainage, because of an increase in the size of the pneumothorax and/or haemothorax, but in all cases drainage was instituted within 24 hours. Haemopneumothorax is the most likely underlying disorder to require delayed drainage. Subcutaneous emphysema is of no value for identifying those cases likely to need delayed drainage. Twenty-four hours is adequate for the observation of patients with slight effects of penetrating wounds of the chest.