Authors report the results of a series of 29 knees examined by magnetic resonance imaging in 16 patients subjected to a long-term corticosteroid therapy (6 systemic lupus erythematosus and 10 kidney transplanted people). Abnormal signals of condyles or tibial plateaus were observed in 12 patients (19 knees). Lesions, which were asymptomatic in 37% of the cases studied, mostly showed (12/24 condyles) a fatty signal area marked out by a hypointense signal strip in T1 edged with a hyperintense signal in T2. Some developed lesions were hypointense whichever the sequence. Our study confirmed the frequency of the lateral condyle involvement (60%) and the bilateral aspect of the lesions (50%) in osteonecrosis of the condyles following the corticosteroid therapy. The associated lesions of articular surfaces (3 cases) could be well assessed through magnetic resonance imaging on T2 or T2* weighted images. In two cases, the abnormal signals of the condyles suggesting a medullary edema (hypointense signal in T1 and hyperintense signal in T2) spontaneously disappeared 6 and 9 months after their discovery. Abnormal signals related to medullary infarcts (10 knees) were always associated with abnormal condyles. Most time their aspect in magnetic resonance imaging, except old calcified lesions, was characteristic: serpiginous hypointense signal isolating areas of fatty signal edged with a hyperintense signal strip in T2. In risk patients, magnetic resonance imaging allows early detecting knee necroses, precising the extent of epiphyseal and metaphyso-diaphyseal lesions and their impact on articular surfaces.