Objective: To analyze the prevalence of osteoporosis during androgen deprivation therapy (A.D.T.). Background: Treatment and prognosis of prostate cancer necessitate management of long-term consequences of A.D.T., including accelerated bone loss resulting in osteoporosis; osteoporotic fractures are associated with excess morbidity and mortality. Patients and methods: Patients with prostate cancer awaiting initiation of A.D.T. were consecutively included from the daily clinic. They were followed every 6 months for 2 years. The study consisted of questionnaires, blood and urine samples and a D.X.A. scan every 6 months and yearly bone scintigraphy. A.D.T. was given as L.H.R.H. agonists, L.H.R.H. antagonists or orchiectomy. None of the patients had received prior A.D.T. or osteoporosis treatment. Results: A total of 105 individuals were included. The mean age was 70 years, median PSA level was 30.5 µg/L and median Gleason score was 7. During the study, the prevalence of osteoporosis rose from 10% at inclusion to 22% after the 2 years of follow-up and the prevalence of normal bone mineral density (B.M.D.) decreased from 32% to 8%. Osteoporotic fractures were prevalent; six patients had a clinical vertebral fracture and two patients had a hip fracture. One patient died after his hip fracture. Conclusion: After 2 years of A.D.T. the vast majority of prostate cancer patients will have osteoporosis or osteopenia. A rigorous observation strategy did not appear to prevent osteoporotic fractures. A new strategy to reduce A.D.T. induced osteoporotic fractures is mandatory.