In this prospective randomized study we evaluated the effect of risedronate, an aminobisphosphonate, on bone mass and turnover in patients who had undergone allogeneic stem cell transplant (SCT) for hematological malignancies. Thirty-four patients (18 females, 16 males, age 32+/-10 years) with bone mineral density (BMD) </=-1.5 SD as a T-score at least 6 months after SCT were treated with calcium 1 g/day and vitamin D 800 IU/day and randomized to receive ( n=17, group 1) or not receive ( n=17, group 2) oral risedronate 5 mg/day. The duration of treatment was 12 months. After 6 months, lumbar BMD increased by 4.4+/-1.6% in patients of group 1 and decreased by 4.3+/-1.5% in those of group 2 ( P<0.05); at the femoral neck, BMD did not change significantly in patients of group 1 (+1.2+/-1.2%), while it decreased in those of group 2 (-4.3+/-2.1%; P<0.05). After 12 months, lumbar BMD further increased (+5.9+/-1.7%, P<0.05), compared to baseline in group 1 and slightly increased (+1.1+/-1.4%) in group 2. No further changes were observed at femoral neck in both groups. In conclusion, treatment with risedronate for 12 months increased BMD significantly at the lumbar spine and prevented further bone loss at the femoral neck in long-term survivors after allo-SCT.