The purpose of this study was to contrast the effects of conventional estrogen treatment with medroxyprogesterone on cancellous and cortical bone change in the first year following premenopausal ovariectomy. This 1-year double-blind randomized therapy trial was stratified by osteoporosis family history and performed in an academic medical center and community hospitals. Premenopausal women 45 +/- 5 years old, postovariectomy for benign diseases were provided 600 mg/day of calcium and randomized to daily therapy with conjugated equine estrogen (CEE, 0.6 mg) or medroxyprogesterone (MPA, 10 mg). The primary outcome variable was spinal quantitative computed tomography (QCT) bone density change over 1 year with additional outcomes of dual-energy X-ray absorptiometry (DXA) of proximal femur (FN), whole body (WB), and spine segment (WBS) and N-telopeptide, bone-specific alkaline phosphatase, and other bone marker, hormonal, and weight changes. Results in the 33 women completing the study, whose initial bone densities were normal (QCT 133 mg/cm3, femoral neck 0.94 g/cm2, whole body DXA 1.13 g/cm2), showed annual QCT loss during CEE therapy of -11.5 mg/cm3 (p < 0.0007) and MPA bone loss of -19.7 mg/cm3 (p 3 SD above premenopausal normal levels) were significantly related to bone loss. Across the year, resorption decreased during CEE but increased on MPA treatment. Significant DXA bone losses were prevented by CEE treatment (-1.4% FN, -.4% WB, and -1.5% WBS, all NS). However, DXA bone loss was not prevented by MPA treatment (-5% FN, -2.8% WB, and -6.1% WBS, all p