Results of pulmonary function studies were compared in two groups of 12 patients each, in whom upper dorsal sympethectomy was performed by the supraclavicular or by the transaxillary approach. Patients were evaluated clinically, radiologically and functionally before operation and again three weeks, three months and six months after denervation. Findings suggest that an increase in small airway resistance concomitant with some degree of pneumoconstriction occurred after upper dorsal sympathectomy by both routes. Musclar transection and possible phrenic nerve retraction damage due to the operative procedure could not be the cause of the above abnormalities because the inspiratory and expiratory forces, inspiratory peak flow and diaphragmatic movement were not significantly reduced after operation by both approaches. However, in a few cases, extrapleural hematomas, segmental atelectasis and relaxation of the daiphrgm could have contributed to the loss of the lung volume. This was evident only in the early period and was obvious in the transaxillary approach group.