The degree of physical activity at the onset of spontaneous pneumothorax was evaluated retrospectively in 219 patients, predominantly smokers who had had this disease for the first time. More than 87% had been inactive at the onset of the symptoms. Moderate exertion was recorded in only 2%, and no patients were exerting themselves heavily when the symptoms began. The occurrence of spontaneous pneumothorax was unevenly (p less than 0.001) distributed over the day. In 9% the symptoms had their onset during quick movement--such as fastening a seat belt--without any effort. The inactivity or low activity at the onset of symptoms of spontaneous pneumothorax suggests that this condition is unrelated to muscle effort.