There is a growing interest to identify genetic factors contributing to the development of erectile dysfunction (ED). Since the early pedigree and twin studies suggested a genetic contribution to ED, a number of follow-up candidate gene studies have strengthened these associations. Candidate gene polymorphisms have been identified in endothelial nitric oxide synthase (eNOS), Angiotensin Converting Enzyme (ACE), the androgen receptor, transforming growth factor-b1 (TGF-b1), and methylenetetrahydrofolate reductase (MTHFR). However, these studies are often difficult to reproduce given the variability between study populations.