Pseudoaneurysms resulting from vascular impingement by osteochondromas are extremely rare. The authors detail the case of a 19-year-old man who represents the third known report in the English literature of a brachial artery pseudoaneurysm associated with a humeral osteochondroma. In patients presenting with a painful upper arm mass and a history of multiple hereditary exostoses, one must have a high index of suspicion for pseudoaneurysm.