Function is the most important consideration in the evaluation of patients with congenital ulnar deficiency. The upper extremity function of 8 patients with ulnar deficiency was evaluated. None of these patients had been treated with surgical procedures directed to the elbow, forearm, or wrist. The functional criteria included: (a) active ranges of elbow, forearm, and wrist motion, (b) power grip, (c) prehension, (d) dexterity, and (e) a patient's activities questionnaire. The averaged total active range of joint motion was 229 or 46% of predicted normal active motion. Power grip averaged 27% of the contralateral extremity, and prehension tests were generally well performed. Timed tests were completed an average of 11.6 s slower than the contralateral control hand. Our patients did not report any deficiencies in bimanual activities. Patients performed most poorly when their congenital anomaly included radiohumeral synostosis or congenital absence, deformity, or contracture of the ipsilateral digits. The radiographic appearances or classification of the ulnar deficiency, in the absence of radiohumeral synostosis. did not correlate well with patient function.