In both sexes, obesity, particularly the abdominal obesity phenotype, may impair fertility. This adverse effect appears to be mainly related to disorders of sex hormone secretion and/or metabolism, leading in turn to a condition of relative hyperandrogenism in obese women and of hypotestosteronemia (and, in some cases, a true hypogonadotropic hypogonadism) in obese men. In women, obesity can also play a relevant role in the pathophysiology of hyperandrogenism and metabolic abnormalities which characterize the polycystic ovary syndrome. These hormonal alterations may also play an important role in the pathophysiology of different obesity phenotypes and associated metabolic and cardiovascular comorbidities. Weight loss can improve hormonal abnormalities and fertility rates in both women and men. Erectile dysfunction, which is very common in obese males, can also be improved by lifestyle intervention strategies favouring weight loss.