RATIONALE: Hormones can stimulate the production of prostate cancer cells. Hormone therapy may fight prostate cancer by reducing the production of androgens. Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. It is not yet known whether hormone therapy plus mitoxantrone and prednisone is more effective than hormone therapy alone for prostate cancer. PURPOSE: This randomized phase III trial is studying hormone therapy, mitoxantrone, and prednisone to see how well they work compared to hormone therapy alone in treating patients who have undergone radical prostatectomy for prostate cancer.