Reliability, accuracy and prognostic value of any classification system are important in evaluation and treatment of femoral head osteonecrosis. The purpose of the present study was to correlate the plain radiographs with MRI in femoral head osteonecrosis. Between 2000 and 2005, 115 hips (72 patients) were evaluated and classified according to the ARCO classification criteria with the use of plain radiographs and additional application of MRI. Classification was performed by consensus between a musculoskeletal radiologist and an orthopaedic surgeon. Sensitivity (SEN), specificity (SP), positive (PPV) and negative (NPV) predictive value of X-rays were estimated. According to MRI, 17 hips were classified as stage I, 25 as stage II, 48 as stage III and 25 as stage IV. The SEN, SP, PPV and NPV of plain radiographs were for stage II 88%, 90.5%, 78.6% and 95%; for stage III 79.2% 82%, 80.8% and 87.2%; for stage IV 76%, 100%, 100% and 90.9%, respectively. The agreement between plain radiographs and MRI was 80.6% for staging the disease, 71.2% for recording the location of the osteonecrotic lesion, 67.1% for evaluating the size of the lesion, 79.2% for the presence of collapse of the articular surface and 56.3% for the degree of collapse. In conclusion, the ARCO classification could miss important information in stages II and III, where treatment aims at preservation of the hip joint integrity. The results of the present study suggest that MRI should be incorporated in the classification of osteonecrosis (stages II and III), to add accuracy and prognostic value.