This prospective randomized, double-blind, placebo-controlled, clinical trial was performed to evaluate the effectiveness of estrogens plus low-dose alendronate on bone metabolism. A total of 150 surgically postmenopausal women with osteoporosis were randomized in three groups: group A, micronized E2 (2 mg/d) plus standard-dose alendronate (10 mg/d); group B, micronized E2 plus low-dose alendronate (5 mg/d); and group C, micronized E2 plus placebo (one tablet per day). In all women, bone mineral density (BMD) and serum bone metabolism markers were assessed at admission and every 6 months for 2 yr. After 2 yr, BMD significantly increased compared with baseline in all groups. The percentage BMD change was significantly higher in groups A and B than in group C. The differences in BMD detected between groups A and B were not statistically significant. Since the 6-month follow-up and throughout the study, serum osteocalcin and bone alkaline phosphatase levels and urinary deoxypyridinoline and pyrilinks-D excretion were significantly reduced in all groups. Serum bone alkaline phosphatase levels significantly decreased in groups A and B, without difference between them, in comparison with group C. In conclusion, in surgically postmenopausal osteoporotic women treated with estrogen replacement, the addition of alendronate at a low dose of 5 mg daily induces a gain of bone mass not significantly different in comparison with that obtained using a standard dose of 10 mg daily.