Prospective Observation Study of Insulin Sensitivity During and After Gonadotropin Releasing Hormone Agonist Therapy for Prostate Cancer

* The study will be divided into 2 groups: one group (Cohort A) will consist of men undergoing GnRH agonist therapy for at least 24 months. The second group (Cohort B) will consist of men undergoing the same treatment but who are scheduled to receive only 6 months of treatment. * The following tests and procedures will be performed when the participant joins the study, then at 3, 6, 12 and 24 months thereafter. (All visits will take place at the General Clinical Research Center (GCRC) at the Massachusetts General Hospital.) Height and weight; blood and urine samples; Oral Glucose Tolerance Test (OGTT), Dual energy X-ray Absorptiometry scan; medical history; and physical examinations.