The investigation on the effects of bone cement on bone was undertaken in the present study. The experiments were conducted on the following 4 group of rabbits, A, B, C, and D. In group A the femoral medullary cavity was reamed, leaving the part of the endosteum of the cortex and bone marrow. In group B reaming of the medullary cavity was extensively performed to remove whole bone marrow and the part of endosteum of the cortex. The medullary spaces produced after the reaming both in group A and B were completely filled with bone cement. In group C, reaming was performed in the same manner as in group B, and the medullary space was kept empty as a control, furthermore, in order to compare the effects of the metal and bone cement on bone intermedullary nails were inserted into the medullary space after the same reaming as in group B. In comparison, the bone specimens were obtained from these 4 groups at intervals after the surgical treatments, the contact microradiographic and histopathological studies were carried out. The most important finding was the action of bone cement causing atrophy and degeneration of bone tissue over a prolonged period of time. The degree of these changes was found to vary depending upon the extent to which the medullary cavity was reamed prior to the introduction of bone cement. In the extensively reamed group, a considerable number of Howship's lacunae were observed along with cancellization of the cortex and, atrophy and degeneration of bone cells, even one year after the treatment. From this study, it was strongly indicated that the effects of bone cement on bone may be caused not only by the heat generated during polymerization, but also by some other factors responsible for the degeneration and atrophy of the bone over a long period of time.