A 60-year-old left-handed woman presented to the emergency department with pain in her left forearm after having been struck by a baseball bat by her partner. On physical examination, the left forearm was swollen and tender, especially with passive pronation and supination. The overlying skin was intact, and the results of neurovascular examination were normal. No other traumatic injuries were present. Radiographs of the left forearm showed an isolated ulnar shaft fracture (upper image, anteroposterior view; lower image, lateral view). Isolated ulnar shaft fractures, also called “nightstick fractures,” result from blunt force trauma to the forearm. The injury classically occurs when the upper arm is raised in self-defense against an attack to the head or torso, such as in intimate-partner violence, as occurred in this case. Comprehensive assessments of the safety and mental health of the patient were performed, and social services were offered. The patient underwent open reduction and internal fixation of the fracture, followed by splinting, although this type of injury may be managed nonoperatively in some cases. At follow-up 10 weeks later, the patient had regained full function of her arm; she had obtained a restraining order for protection and was receiving assistance from a social worker.