We describe a case of Horner syndrome occurring secondary only to the high insertion of a chest tube after video-thoracoscopic apicectomy for spontaneous pneumothorax. Because all other causes could be ruled out, the Authors assume that the lesion to the sympathetic nerve fibres was caused by pressure exerted by the tip of the chest tube. Horner syndrome due to this cause can easily be avoided. The tip of the chest tube should be kept at the level of, or below, the third posterior rib, unless the clinical situation dictates otherwise. This cause of Horner syndrome must be taken into account because, if recognized and treated promptly, it can be fully resolved.