OBJECTIVE: To measure changes in spinal and femoral neck bone mineral densities in patients treated for Graves thyrotoxicosis. DESIGN: Cohort study. SETTING: Tertiary care center. PATIENTS: Fifteen women with active Graves thyrotoxicosis. Six patients were premenopausal and nine were postmenopausal. All patients had evidence of thyrotoxicosis as indicated by a raised total serum thyroxine, suppressed serum thyroid-stimulating hormone, and an elevated technetium-99m pertechnetate thyroid scan. A control group of 15 healthy volunteers matched for age, sex, and menopausal status were followed during the same period. MEASUREMENTS: Bone mineral density was measured by dual-energy x-ray absorptiometry at baseline and after 12 months of antithyroid therapy when euthyroidism had been achieved. RESULTS: After 12 months of therapy, total serum thyroxine, total serum triiodothyronine, serum alkaline phosphatase, and serum bone Gla-protein activities had returned to normal in all patients (P < 0.001 for all comparison between initial and final biochemical measurements). During this interval, the mean lumbar spine bone mineral increased from an initial value of 1.01 g/cm2 to 1.07 g/cm2, an increase of 6.6% per year (95% CI, 3.6% to 9.6%) (P < 0.001 compared with controls). Increases in femoral neck (1.2%/y; CI, -2.1% to 4.5%; P = 0.2 compared with controls) and femoral trochanter bone mineral (3.2%/y; CI, 2.4% to 8.7%; P = 0.2 compared with controls) were not statistically significant. Using forced-entry multiple regression analysis, the severity of the thyrotoxicosis was independently associated with the percentage increment in lumbar spine bone mineral density after 12 months of antithyroid therapy. CONCLUSION: Effective treatment of Graves thyrotoxicosis was associated with increases in lumbar spine and femoral neck bone mineral. Although the changes in bone mineral were modest, our data suggest that thyrotoxic bone loss may be a reversible disorder.