Six cases of aneurysm and thrombosis of the ulnar artery in the hand occurred. Based on a review of the clinical history and examination, surgical findings, and pathology, it appears that the initiating event in the development of aneurysms and thromboses of the ulnar artery was intimal injury associated with trauma. In addition to a description of the pathophysiology, three clinical groups are defined; hypothenar mass with or without Raynaud's phenomena, and Raynaud's phenomena without hypothenar mass. A positive Allen's test and absence of ulnar nerve palsy were important diagnostic features. Based on the clinical and pathophysiological findings, surgical excision of the lesion is considered to be the preferred method of treatment.