Aseptic osteonecrosis of the medial femoral condyle has recently been reported as a complication of arthroscopic surgery. The time interval between the onset of symptoms and pathognomonic MRI changes (diagnostic window) is not known for osteonecrosis of the knee. To determine the prevalence of early-stage spontaneous osteonecrosis of the knee (SONK) we prospectively examined 176 patients by MRI between May 1998 and December 1999. In six patients MRI revealed a bone marrow edema pattern and subtle subchondral bone changes in the medial condyle consistent with early-stage SONK (prevalence of 3.4%); in the 53 patients older than 65 years the prevalence was 9.4%. In 10 patients (5.7%) the bone and marrow changes on MRI imaging either resolved on follow-up MRI and were regarded as transient epiphyseal lesions or were considered to be reactive changes due to underlying degenerative articular disease. Including MRI in the preoperative diagnostic procedures could avoid missing the diagnosis of avascular necrosis before planning an operative treatment of suspected meniscal tears in elderly patients.