Background: Posterolateral rotatory instability is a type of ulnohumeral instability seen following elbow trauma. It is caused by a deficiency in the lateral collateral ligament complex that allows the radius and ulna to subluxate as a single unit with respect to the distal part of the humerus. There are few studies on this type of instability in children. Our purpose was to evaluate cases of posterolateral rotatory instability in children to better understand its presentation and manifestation as compared with those in adults. Methods: This was a retrospective chart review of patients from three academic centers. Eligible for inclusion were patients with a diagnosis of posterolateral rotatory instability who were treated with lateral ulnar collateral ligament reconstruction when they were less than nineteen years of age. Results: Nine patients met the inclusion criteria. The mean age at the initial injury was ten years, and the average time from the initial injury to the final operation was 3.7 years. Six patients had prior elbow dislocation, and three had an isolated elbow fracture. Six of the nine patients had a forearm or elbow contracture. Only one patient had a positive pivotshift test during the preoperative office examination, but all had a positive pivot-shift test when examined under anesthesia. Six had radiographic evidence of posterolateral rotatory instability. All patients underwent lateral ulnar collateral ligament reconstruction. At the time of follow-up, at a minimum of one year after the ligament reconstruction, there was no evidence of deformity secondary to early physeal closure and all elbows remained stable. Conclusions: Although posterolateral rotatory instability of the elbow is rare, it does exist in children. The instability may not always be recognized because of masking by contracture but, as is the case with adult patients, radiographs may show evidence of the instability. In children with contracture, the clinician should consider the possibility of a masked posterolateral rotatory instability and plan accordingly at the time of contracture release. Surgical correction is technically difficult, and traditional ligament reconstruction in skeletally immature patients may pose a risk to the lateral humeral condylar and epicondylar physes. Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.