The aim of this study was to evaluate bone mass changes after 1 year of four different types of pharmacological intervention. Ninety-seven prostate cancer patients treated with androgen deprivation therapy, and severe osteopenia or osteoporosis were retrospectively studied. Patients were divided in four groups. Group 1: 28 patients treated with denosumab, Group 2: 24 patients treated with alendronate, Group 3: 24 patients with no antiresorptive treatment and Group 4: 21 patients previously treated with alendronate and switched to denosumab. Dual X-ray absorptiometry was performed at baseline and after 1 year. Bone mass changes at the L2-L4 lumbar spine, femoral neck and total hip were evaluated. No differences were found at baseline. After 1 year, men receiving denosumab or alendronate (Group 1 and 2) showed a significant bone mass increase at the lumbar spine (+2.4 and +5.0 %, respectively), while no significant changes were observed in Group 3 and 4. At the femoral neck, Group 1 and 2 patients showed a significant bone mass increase (+3.7 and +3.6 %, respectively), while no significant changes were observed in Group 3 and 4. At the total hip, we observed a significant bone mass increase in Group 1 (+2.9 %) and a significant bone mass loss in Group 3 patients (-1.9 %). No significant changes were observed in Group 2 and 4. Denosumab increased significantly bone mass in all three dual X-ray absorptiometry standard sites, while alendronate did not at total hip. No benefit was observed in men previously treated with alendronate who switched to denosumab treatment.