Hypothenar hammer syndrome (HHS) is characterized by digital ischemia after repetitive traumatic injury to the ulnar artery. Some hypothesize that susceptible patients have an intrinsic vasculopathy such as fibromuscular dysplasia (FMD). To investigate this hypothesis, we reviewed our institutional experience with this syndrome over 25 years. Clinical records were reviewed from all patients who underwent surgical treatment for HHS (1987 to 2011), and histologic features of excised ulnar arteries were characterized. A total of 67 patients (mean age 45 y; range, 21 to 75 y; 65 men) were treated for unilateral or bilateral disease. Common symptoms included digital pain (96% of cases), cold intolerance (79%), cyanosis (70%), numbness (54%), tingling (51%), and ulceration (40%). Angiography showed ulnar artery occlusion (89%), irregularity (56%), tortuosity (46%), and digital emboli (89%). Common pathologic features (from 75 specimens) included: luminal thrombosis (87%); intimal thickening (60%) and fibrosis (57%); internal elastic membrane disruption (95%); medial fibrosis (96%), hypertrophy (43%), neovascularization (49%), dilatation (29%), and disruption (25%); and adventitial neovascularization (53%). Specific features of dysplasia were present in 10 cases (15%), including smooth muscle disorganization and additional smooth muscle bundles outside the external elastic membrane, but typical FMD was not identified. Histologic features in HHS most often represent secondary changes consistent with repetitive trauma. Dysplastic features can be found in occasional cases, but FMD does not appear to contribute to HHS in most patients. Angiography should be interpreted with caution in superficial locations, as a string-of-beads appearance may simply reflect a posttraumatic corkscrew deformity of the ulnar artery.