The objective of this study was to quantitatively investigate the long-term radiologic and functional outcomes of antegrade intramedullary (IM) nailing through the olecranon apophysis for ulnar fractures in skeletally immature patients. A retrospective review was conducted of skeletally immature patients with ulnar fractures that had antegrade IM nail fixation through the olecranon apophysis. Patients were excluded if they had a previous forearm fracture or a fracture of the contralateral forearm. Functional measures included the Activities Scale for Kids (ASK) questionnaire for patients younger than 15 years of age and the Disabilities of the Arm, Shoulder and Hand (DASH) questionnaire for patients older than 15 years of age. Forearm range-of-motion measurements were collected from the fractured forearm and the contralateral control forearm. Radiologic outcomes were evaluated for ulnar, olecranon, coronoid, and trochlear notch proportions. Nineteen patients were evaluated. The average age at operation was 10.8 years (range 1.6-15.9) and the mean follow-up time was 3.4 years (range 1.2-7.2). Nine patients completed the ASK questionnaire, with a mean score of 93.9 (range 68.9-100), and 10 patients completed the DASH questionnaire, with a mean score of 6.0 (range 0-35). The mean fractured forearm supination was 103.2 degrees compared with 109.2 degrees on the control forearm (P < 0.05). Furthermore, the mean fractured forearm trochlear notch width was 17.7 mm compared with 17.1 mm on the control forearm (P < 0.05). Similarly, the mean fractured forearm trochlear notch height was 16.5 mm compared with 17.0 mm on the control forearm (P < 0.05). This retrospective review suggests that antegrade IM nail fixation through the olecranon apophysis for ulnar fractures in skeletally immature patients is a safe procedure. There is no significant ulnar length disruption or functional limitations, despite the differences in trochlear notch measurements and supination between the fractured and control forearms. Overall, IM nail fixation through the olecranon apophysis for surgically indicated ulnar fractures has minimal outcome limitations, with no evidence of prospective growth disruption. A larger randomized prospective trial should be conducted to strengthen the evidence of this study.