The present randomized, double-blind, placebo controlled, 2-year study is the first to evaluate the effect of 1.25 and 2.5 mg tibolone daily oral administration on trabecular and cortical bone loss in early postmenopausal women. Ninety-four healthy, normal weight, nonsmoking women participated 1-3 years following spontaneous menopause. Twenty-three subjects were randomized to the placebo group, 36 to the 1.25 mg/day tibolone group, and 35 to the 2.5 mg/day tibolone group. Bone density was assessed at 6 month intervals. Spinal trabecular bone density (BD) was measured with quantitative computed tomography. Phalangeal cortical BD was measured by radiographic absorptiometry. The 2-year change vs, baseline in the placebo group for trabecular BD was -6.4% (95% confidence interval -8.1 to -4.7). Cortical BD did not change significantly. At 24 months both tibolone groups showed a statistically significantly higher trabecular [9.4% (6.6-12.2) for the 1.25 mg group and 14.7% (11.8-17.5%) for the 2.5 mg group] and phalangeal BD [4.4% (1.5-7.4) for the 1.25 mg group and 6.8% (3.8-9.8) for the 2.5 mg group] as compared to the placebo group. After 2 years of tibolone in both regimes, trabecular and phalangeal BD was significantly higher as compared to pretreatment values. At 24 months the 2.5 mg group showed a significantly higher trabecular (p < 0.001) but not phalangeal (p = 0.064) BD compared to the 1.25 mg group. Tibolone prevents early postmenopausal bone loss by inducing an increase in trabecular and phalangeal BD.