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Avascular necrosis (AVN) is characterized by death of both trabecular bone as well as bone marrow elements. Weight-bearing bone becomes mechanically weakened and may eventually collapse, secondarily leading to osteoarthritis and debilitating pain. Early diagnosis and treatment of this entity are crucial because it affects relatively young individuals, and treatment options for advanced disease are limited. Magnetic resonance imaging (MRI) has emerged as the modality of choice for the evaluation of avascular necrosis of bone. We will discuss applications of MRI for early diagnosis, for monitoring therapy, and for its potential role in assessing individuals at risk of AVN. Although bone scintigraphy using single photon emission computed tomography (CT) may be nearly as accurate as MRI, MRI offers a more specific diagnosis in the patient who presents with hip pain of uncertain etiology. In addition, lesion size and location can be more easily assessed on magnetic resonance images, and this has been shown to relate to prognosis and need for treatment. We will review the pathophysiologic mechanisms of AVN and the current use of MRI in the diagnosis of this condition.