Since the development and growth of the prostate cancer is highly dependent on androgens, androgen deprivation therapy continues to be the treatment of choice for patients with advanced prostate cancer. The therapy is very effective, but responses are often short-lived. We describe here a novel molecular mechanism that may explain why prostate cancer cells become resistant to hormonal therapy. Amplification of the androgen receptor (AR) gene was found to be selected for during androgen deprivation therapy in 23% of prostate cancer patients who experienced local tumor recurrence. Amplification leads to increased expression of the AR gene, which enables the cancer cells to more effectively utilize the residual low levels of androgens for sustaining cell growth. Discovery of AR amplification as a possible molecular mechanism of therapy resistance in prostate cancer should prove useful for development of more effective endocrine therapy regimens as well as diagnostic and predictive tests for therapy failure.