BACKGROUND Spontaneous osteonecrosis of the knee (SONK) is a clinical entity identified by acute knee pain usually associated with joint effusion, with radiographic findings of a radiolucent defect on the weightbearing area of the femoral condyle. Conservative treatment is initially undertaken; however, surgical procedures are often necessary. Historically, surgical options have included core decompression, cartilage repair, high tibial osteotomy, or joint arthroplasty. Few studies in the literature have reported the use of fresh osteochondral allograft (OCA) for the treatment of SONK lesions. OBJECTIVE OCA transplantation is an effective treatment for SONK lesions on the medial femoral condyle. METHODS Case series; Level of evidence, 4. METHODS A case series was analyzed of 7 patients treated with OCA for large SONK lesions of the medial femoral condyle with a minimum 4-year follow-up. All patients experienced failure of at least 6 months of conservative treatment and declined arthroplasty as the form of definitive treatment for medial femoral condyle lesion. All patients underwent OCA of the medial femoral condyle. Mean lesion size was 4.6 cm2 (range, 3.24-6.25 cm2), with a mean condylar width of 41.7 mm (range, 35.4-48.6 mm), resulting in a median proportion (lesion size/condylar width) of 56.8% (range, 32.7%-62.6%). The median surface allograft area was 5.1 cm2 (range, 3.2-6.3 cm2). RESULTS The median follow-up was 7.1 years (range, 4.5-14.1 years). No patient had additional surgery following OCA transplant; the allograft failure rate was 0%. Subjective outcome scores from the International Knee Documentation Committee, Knee injury and Osteoarthritis Outcome Score, and modified Merle d'Aubigné-Postel scale improved from preoperative assessment to the latest follow-up. All patients were extremely satisfied with the results of the OCA transplant. CONCLUSIONS Fresh OCA transplantation demonstrated excellent efficacy, durability, and satisfaction in this group of patients with isolated stage 2 and 3 SONK lesions who had experienced failure of conservative treatment. Fresh osteochondral allografts are an attractive method for surgical management of selected patients with spontaneous osteonecrosis of the knee.