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Eleven hips in nine patients with Perthes' disease were studied by plain radiography at 3-month and MRI at 6-month intervals over a period of 2 years. The aim was to clarify the value of MRI in estimating epiphyseal involvement and in predicting uncoverage of the epiphysis. Signal intensities of the epiphysis and metaphysis were visually evaluated from T1- (T1W) and T2-weighted (T2W) images. The extent of decreased signal intensity (DSI) in the epiphyses was volumetrically calculated from T1W images and then compared with follow-up radiographs. The area of epiphyseal DSI corresponding best with Catterall's classification was seen by MRI 3-8 months after the first symptoms. MRI images obtained earlier usually showed less involvement than the follow-up radiographs. However, two features predicting extensive epiphyseal necrosis were: (1) DSI on both T1W and T2W images covering over two-thirds of the epiphysis and (2) diffuse bone marrow oedema of the femoral neck and metaphysis. When T1W images showed a reappearance of high signal intensity patches in the lateral quarter of the epiphysis, no clinically significant uncoverage was seen during the follow-up. Extensive epiphyseal necrosis can, therefore, sometimes be predicted by MRI even within the first 3 months, but MRI visualises epiphyseal involvement more clearly 3-8 months after the first symptoms.