This study investigated whether bone turnover influences the response to alendronate in women with postmenopausal osteoporosis. One hundred postmenopausal osteoporotic women were randomized to receive either alendronate (10 mg/day) plus calcium (1000 mg/day) (n = 50) or calcium alone (n = 50). Vertebral and radial bone density, measured by DXA, and markers of bone turnover were assessed at baseline and after 1 and 2 years. At the end of treatment, alendronate users showed an increase of 5.0% and 2.3%, respectively, at the lumbar spine and ultradistal radius; in the group treated only with calcium, bone mineral density (BMD) decreased by 1.6% at the lumbar spine and 1.3% at the ultradistal radius. The difference between the two groups was significant (P < 0.001). The patients were divided into high (HT) or low (LT) bone turnover groups, as assessed by 24-hour whole body retention (WBR%) of (99m)Tc-methylene-diphosphonate. The response to alendronate treatment was greater in HT patients compared with LT patients. In fact, at the end of the study period, BMD at the lumbar spine had increased by 7.9% in HT patients and by 3.0% in LT patients; the difference between the two groups was significant (P < 0.001). No significant difference between the two groups was found for BMD at the ultradistal radius. In conclusion, the present study demonstrates that 2-year treatment with alendronate has highly positive effects on bone mass at both the lumbar spine and ultradistal radius. The increase in bone mass, especially at the axial level, is influenced by bone turnover. Therefore, the evaluation of bone turnover may be useful in predicting the response to alendronate treatment.