Purpose Androgen deprivation therapy may promote the development of the metabolic syndrome in patients with prostate cancer. We assessed the prevalence of the full metabolic syndrome and its components during the first year of androgen deprivation therapy. Materials and Methods This observational, multicenter, prospective study included 539 patients with prostate cancer scheduled to receive 3-month depot luteinizing hormone-releasing hormone analogs for more than 12 months. Waist circumference, body mass index, lipid profile, blood pressure and fasting glucose were evaluated at baseline and after 6 and 12 months. The metabolic syndrome was assessed according to NCEP ATP III criteria (2001) and 4 other definitions (WHO 1998, AACE 2003, AHA/NHLBI 2005 and IDF 2005). Results At 6 and 12 months after the initiation of androgen deprivation therapy, significant increases were observed in waist circumference, body mass index, fasting glucose, triglycerides, total cholesterol, and high-density and low-density lipoprotein cholesterol. No significant changes in blood pressure 130/85 or greater were detected. A nonsignificant increase of 3.9% in the prevalence of the full metabolic syndrome (ATP III) was observed (22.9% at baseline vs 25.5% and 26.8% at 6 and 12 months, respectively). The prevalence of the metabolic syndrome at baseline varied according to the definition used, ranging from 9.4% (WHO) to 50% (IDF). At 12 months significant increases in prevalence were observed with the WHO (4.1%) and AHA/NHLBI (8.1%) definitions. Conclusions Androgen deprivation therapy produces significant early effects on waist circumference, body mass index, fasting glucose, triglycerides and cholesterol. The prevalence of and increase in the metabolic syndrome depend on the defining criteria. Counseling patients on the prevention, early detection and treatment of specific metabolic alterations is recommended.