Summary.After a brief account of the disorders in which pneumothorax and mediastinal and cutaneous emphysema may occur the author gives a survey of the theoretically possible mechanisms which may operate in the production of these conditions after tracheotomy. He divides the complications into two groups:Direct tracheotomy complications, due to the operation itself, andindirect tracheotomy complications, the ultimate cause of which must be sought for in the existing primary disease, but in which the tracheotomy must be assumed to play a contributory rôle.Three cases observed by the author himself are described, in each of which a patient with diphtheria got tension‐pneumo‐thorax, mediastinal emphysema and cutaneous emphysema in the course of the first hours following the operation, after an interval of relatively good condition.While the diagnosis in the first case was established only on post‐mortem examination, in the two last cases the condition was diagnosed intra vitam by direct measurement of the pressure in the pleural cavity, as radiographic examination could not be carried out. The two last patients were treated by exsufflation, and one of them recovered. The other had diffused diphtheric changes right down in the finer branches of the bronchi.The author discusses the frequency of the complications and, in common with other authors, he is inclined to think that the condition is of more frequent occurrence than would seem to be suggested by the reports to be found in the literature.As regards the age distribution it is pointed out that the complications seem in marked degree to belong to the years of childhood, seeing that there have not been reported any cases of mediastinal emphysema and pneumothorax after tracheotomy in patients over 8 years old. Of 36 patients 27 were under the age of 4 years.The prognosis in cases of pneumothorax seems to depend upon the mechanism of production, since all patients with pneumothorax due to direct injury to the pleura during the operation are seen to have recovered, whereas the prognosis in pneumothorax occurring as a so‐called indirect tracheotomy complication is very unfavourable, at any rate in the absence of treatment.In conclusion the different pathogenetic possibilities are discussed.The author believes that the direct tracheotomy complications may generally speaking, be regarded as less dangerous. The only possibility for the occurrence of pneumothorax in this group is deemed to lie in a direct injury to the pleura during the operation.In the indirect tracheotomy complications the cutaneous emphysema is merely to be regarded as an incidental phenomenon in an existing mediastinal and interstitial emphysema. Here the conditions are favourable for the development of pneumothorax, a valvular and, perhaps, tension‐pneumothorax, which may rapidly lead to death.Repeated exsufflations is presumed to be the only method of treatment which is practically feasible in case of these unruly little patients, where continuous drainage presents great difficulties.In the cases here described there seems to have existed a combination of both mechanisms: cutaneous emphysema occasioned by the forcing out of air from the tracheotomy‐opening when the cannula had fallen out or become choked up, and afterwards mediastinal emphysema arising through further transmission of an interstitial emphysema, and pneumothorax due to rupture of subpleural emphysematous vesicles.In conclusion the author seeks to give an explanation of the apparently more frequent occurrence of pneumothorax and mediastinal emphysema in tracheotomized than in non‐tracheo‐tomized patients with diphtheric or other stenotic processes in the tr