PURPOSE: A deeper investigation of medical and musculoskeletal conditions in patients with ulnar longitudinal deficiency (ULD) is needed. The association between the severity of the manifestations of ULD in the hands and forearms has not been firmly established. The purpose of this study was to describe the medical and musculoskeletal conditions associated with ULD and examine the relationship between hand and forearm anomalies. METHODS: The Congenital Upper Limb Differences registry was queried for all patients with a diagnosis of ULD, as defined by the Oberg-Manske-Tonkin classification system, between 2014 and 2020. The patients' demographic information, medical and musculoskeletal comorbidities, radiographs, and clinical images were reviewed. The participants were classified using the Bayne, Cole and Manske, and Ogino classification systems. RESULTS: Of 2,821 patients from the Congenital Upper Limb Differences registry, 75 patients (2.7%) with ULD (14 bilateral), with 89 affected extremities, were included. Hand anomalies were present in 93% of the patients. Approximately 19% of the patients had an associated medical comorbidity, and 20% of the patients had an associated musculoskeletal condition. Cardiac anomalies were present in 8.0% of the patients, and 12% of the patients had a lower extremity abnormality. Radial head dislocation was observed in 13 of 18 patients with Bayne type II or III ULD compared with 8 of 43 patients with other types of unilateral ULD. There was a significant positive association among the Bayne and Ogino, Bayne and Cole/Manske, and Ogino and Cole/Manske classification systems in patients with unilateral ULD. CONCLUSIONS: Associated medical and musculoskeletal conditions are common in patients with ULD, of which cardiac and lower extremity abnormalities are most frequently observed. There is a significant positive association between the severity of forearm anomalies and that of hand anomalies in patients with unilateral ULD. All patients with ULD should undergo a thorough cardiac evaluation by their pediatrician or a pediatric cardiologist. TYPE OF STUDY/LEVEL OF EVIDENCE: Symptom prevalence study III.