Occlusion of the superficial palmar branch of the ulnar artery amongst workers in mechanical workshops is related to the use of the hand as a hammer. Of 79 habitual hammerers, 11 (14%) showed evidence of ulnar artery occlusion in one or both hands. Those suffering from the hypothenar hammer syndrome were found to be older and to have had longer employment in hammering jobs, than those who used their hands as hammers but had intact ulnar arteries. Symptoms of vascular insufficiency were universal among those with blocked ulnar arteries. It should be possible to prevent the lesion from occurring by insistence on good workshop practice.