Osteonecrosis (avascular necrosis, AVN, aseptic necrosis) is the consequence of the disturbance of blood supply to specific bone area. AVN occurs as a result of many different conditions such as fracture, thrombosis, mechanical vessel injury, fat embolism and many others. The most frequent location is the femoral head, but aseptic necrosis can also affect epiphysis of long bones and small bones of upper and lower limb. Avascular necrosis is also one of the serious long-term complications of antineoplastic treatment, including high dose steroids, specific cytostatics and radiotherapy. Children and adolescents treated for hemato-oncologic diseases during the intensive bone growth are at special risk for osteonecrosis. Adequate management is based on early and proper diagnosis and treatment - conservative, pharmacological or surgical. MRI technique has been shown to have the best sensitivity and specificity in the moment of the diagnosis of AVN, as well as during the disease proccess.