This review has revealed a strong association between the BDNF Val66Met polymorphism and weight fluctuations in patients with psychiatric disorders, specifically including schizophrenia, eating disorders (such as anorexia nervosa and bulimia nervosa), bipolar disorder, major depressive disorder, obsessive-compulsive disorder (OCD), and related conditions. The gene discussed is BDNF; the disease is obsessive-compulsive disorder.