Background: Osteonecrosis has been reported as a rare complication of arthroscopic knee surgery, with the diagnosis based on radiographic findings associated with recurrent or worsening clinical symptoms. The term osteonecrosis has been applied to this clinical entity despite a lack of pathologic evidence to support the diagnosis. The purpose of this study was to document the pathologic changes involved in this rare process. Methods: Patients who had undergone an arthroscopic procedure followed by a total knee replacement within two years were eligible for the study. Inclusion criteria included pre-arthroscopy magnetic resonance imaging findings consistent with a meniscal tear with otherwise normal bone morphology followed by a provisional diagnosis of post-arthroscopy osteonecrosis based on subsequent imaging studies. Patients were excluded if a laser-assisted device had been utilized during the arthroscopy. Seven patients (eight knees) with an average age of sixty-four years met the criteria and were included in the study group. Results: All patients had undergone an arthroscopic medial meniscectomy, and two also had had a chondroplasty, with use of a mechanical shaver. Seven of the post-arthroscopy lesions involved the medial femoral condyle, and one lesion involved the medial tibial plateau. Pathologic analysis revealed a subchondral fracture with callus formation, indicated by the presence of woven bone, in all cases. Four patients had essentially intact articular cartilage overlying the lesion, which was characterized by disruption of the trabecular architecture indicative of subchondral bone fracture. The other four patients had an isolated osteochondral defect with reparative tissue within the base of the defect. Only two knees had localized evidence of osteonecrosis, which appeared to be secondary to the fracture. Conclusions: This study provides pathologic evidence supporting the concept that subchondral fracture, and not osteonecrosis, is the major event in this rare complication following arthroscopy. Further investigation into the etiology of this condition is warranted. Level of Evidence: Prognostic Level IV. See Instructions to Authors for a complete description of levels of evidence.