Osteonecrosis of the medial femoral condyle rarely appears after an arthroscopic meniscectomy. However, in such cases, MRI is very important for early diagnosis. Here, we report two cases of osteonecrosis, which were not found in a preoperative MRI examination, but were then diagnosed after meniscectomy. Although the etiology of osteonecrosis still remains unclear, microcirculation disorders and microfracture of the femoral condyle are believed by many investigators, and varus deformation of the knee joint caused by meniscectomy are considered to be risk factors causing osteonecrosis. Middle-aged and elderly patients are also particularly at risk. Such patients who undergo meniscectomy should be carefully observed, considering that osteonecrosis might appear postoperatively.