Twenty patients with "idiopathic" bone infarction were studied. There were 18 men and 2 women, age 30 to 69 years, at the time of the diagnosis (mean age = 49 years). Sixty-five lesions were recorded with a marked predominance to the lower extremities (77 p. cent are located around the knees) and to the metaphysis (only three pure diaphyseal lesions). Considered asymptomatic, these lesions were painful in 6 patients. They are multiple, and in this case symmetrical, in 12 patients (60 p. cent). X-Rays disclose the classic heterogeneous ball-like, smoke twirled or encapsulated calcifications. A periosteal thickening opposite the lesions was observed in 6 patients; this may be the only radiological sign. Finally, 18 of the 65 lesions were not visible on standard X-Rays, and obvious on MRI. The MRI aspect is characteristic and may be superposed on the basic lesions already described in the course of aseptic osteonecroses of the femoral head. This is, besides, the most sensitive test, snowing a larger number of more extended necroses than the other examinations. CT scanning as well as scintigraphy present a limited advantage. More than half of the patients also present epiphyseal aseptic osteonecroses, often multiple (55 sites for 13 patients), and often unrecognized. The etiological factors are in fact common to both of these diseases: steroid therapy, alcoholism, dyslipidemia, idiopathic forms. Among the possible causes, the literature mentions lupus erythematosus, renal transplant, cytosteatonecrosis, arteriopathies while there is no post-traumatic form. All of the characteristics of these bone infarctions determine a topographic form of the osteonecrotic disease.(ABSTRACT TRUNCATED AT 250 WORDS)