Authors measured the bone marrow pressure(B.M.P.) of 59 femoral heads in cases of either suspicious or diagnosed osteonecrosis and obtained the significant relationship between increased B.M.P. and the osteonecrosis. Increased bone marrow pressure provided both predictive and early diagnostic importance in even the preclinical stage of evolution of the disease, which subsequently had core biopsy proved osteonecrosis of femoral head. The results obstained were as follows; 1. Roentgenographically apperent osteonecrosis showed abnormal B.M.P. pattern, which were proved by the core biopsy. 2. In 21 cases of preclinical osteonecrosis, which did not show roentgenographic changes of osteonecrosis, measured B.M.P. parterns indicated as early stage of osteonecrosis in core biopsy at all. 3. The incidence of abnormal pressure pattern I was 51% and that of pattern II was 30%. 4. Enneking radiological staging did not correlate with the results of B.M.P. 5. The measurement of B.M.P. is relatively easy and simple method without any remarkable risk. The results of this study had great meaning in emphasizing the contribution of the bone marrow pressure measurement dianosing the preclinical stage of osteonecrosis, which had no roentgenographical changes and diagnostic symptoms.