Summary Ninety-four cases of spontaneous pneumothorax are reviewed. The age, sex and population incidences are given or estimated. There is no relationship to smoking nor effort. The mode of onset and associated illnesses are recorded. Difficulties in diagnosis are enumerated. Dyspnœa is the cardinal symptom, with chest pain and cough next in importance. The pulse rate and white cell count are likely to be raised. The pleural effusion was eosinophilic in all cases where a differential white cell count was performed on the pleural fluid. The length of stay in hospital and the recurrence rates are reviewed. The complications of different methods of treatment are reviewed. The cause of the disease is not known in a large proportion of the cases.