Abstract Details are given of a case in which reoperation for upper-limb sympathectomy was carried out 7 years after the first operation. On each side the returned sympathetic activity was abolished by a removal of tissue from the area of the previous operation and without the production of Horner's syndrome. Gustatory sweating, which had developed after the first operation, was abolished by the second. These observations are discussed, particularly in relation to the writer's earlier tests with paravertebral blocks. It is concluded that the fibres responsible for sympathetic activity returning to the hand after sympathectomy arise from a regrowth of fibres sectioned at the operation. The writer's operation, designed to make functioning reconnexion more difficult, is more effective than the usual procedures.