To evaluate the effects of 1α-hydroxyvitamin D₃ (1α(OH)D₃), a series of clinical trials, preventive and therapeutic, were performed in an open label manner in women immediately after oophorectomy. The series included a total of 121 oophorectomized subjects, whose lumbar bone mineral density (L_2-4BMD) was followed by the use of dual energy X-ray absorptiometry. (1) Preventive trial: 61 women who had undergone premenopausal bilateral oophorectomy, were divided into 3 groups (Group C: control; Group L: 0.25μg 1α(OH)D₃/day; Group H: 0.50-0.75μg 1α(OH)D₃/day). The changes in BMD and chemical indices were followed up for one year. (2) Therapeutic trial: the trial included 60 premenopausally oophorectomized subjects having L_2-4BMD lower than the normal control level minus 1SD which has been reported in age-matched normal Japanese women. These subjects were divided into 3 groups and treated in the same way as in the preventive trial. In the preventive trial, L_2-4BMD decreased by 8.2%, 6.5% and 4.5% in groups C, L and H, respectively, at 12 months of treatment, whereas in the therapeutic trial, L_2-4BMD decreased by 3.6%, 3.2% and 0.8% in the groups C, L and H, respectively, at 12 months of treatment. In conclusion, 1α(OH)D₃ was found to be effective both to prevent the bone loss subsequent to bilateral oophorectomy and improve low bone mass after oophorectomy.