OBJECTIVES: The purpose of this study is to evaluate the efficacy of annual zoledronic acid treatment in Japanese patients with nonmetastatic prostate cancer during androgen deprivation therapy (ADT). METHODS: This is a single institution 12-month study. Between 2016 and 2019, patients aged 70 years or older on ADT for nonmetastatic prostate cancer had bone mineral density (BMD) measured and 10-year probability of fracture calculated using fracture risk assessment tool (FRAX). Patients who showed osteopenia or had a 10-year hip fracture risk ≥ 3% or a 10-year probability of major osteoporotic fracture ≥ 20% were offered treatment with zoledronic acid 5 mg intravenously (ZA group). The patients who did not receive treatment were set as the control group. Lumbar and hip BMD were measured 6 and 12 months after treatment in the ZA group and 12 months after baseline in the control group. The yearly BMD change of both groups was compared. RESULTS: The mean ages of the ZA group (n = 26) and control group (n = 12) were 80.5 ± 9.1 and 76.1 ± 6.7 years, respectively. In the ZA group, lumbar and hip BMD changes at 12 months were +2.1% and +0.8%, respectively. In the control group, lumbar and hip BMD changes were −0.9% and −4.9%, respectively. There were statistically significant differences between the 2 groups in BMD percent changes (P < 0.05). CONCLUSIONS Without intervention, BMD tends to continue to decrease during ADT. Our findings suggest that administration of zoledronic acid enables maintenance of BMD in the older adults.