To evaluate the MR imaging findings of early stage spontaneous osteonecrosis of the knee. This study includes 21 spontaneous osteonecrosis cases (12 men, 9 women; mean age, 56.9 years) consulting to a radiology clinic for MR imaging of knee joint. Antero-posterior and lateral knee radiographies of all patients were obtained. 25 lesions observed on MRI in 21 patients were analysed. MR images were analysed for the localization and dimension of the lesion, the presence of hypointensity in the subchondral bone, the presence of focal contour depression and the presence of additional meniscal pathology. As a means of medical treatment intravenous iloprost was applied. There was no specific pathological finding on plain radiography. In 16 cases, the lesions were localized on the medial femoral condyle. Nine lesions were defined as small sized (36 %), and 16 as large (64 %). Subchondral hypointense focus was seen in 11 lesions. Contour depression was found on medial weight bearing joint surfaces in 2 cases. In 15 cases, grade 2 meniscal degeneration was found on the posterior horn of the medial meniscus. MR imaging can be applied as an effective modality for the diagnosis of early stage spontaneous osteonecrosis.