This study evaluated associations between body mass index (BMI) and psychiatric disorders. Data from 41,654 respondents in the National Epidemiologic Survey on Alcohol and Related Conditions were analyzed. After controlling for demographics, the continuous variable of BMI was significantly associated with most mood, anxiety, and personality disorders. When persons were classified into BMI categories of underweight, normal weight, overweight, obese, and extremely obese, both obese categories had significantly increased odds of any mood, anxiety, and alcohol use disorder, as well as any personality disorder, with odds ratios (ORs) ranging from 1.21 to 2.08. Specific Diagnostic and Statistical Manual of Mental Disorders-revision IV mood and personality disorders associated with obesity included major depression, dysthmia, and manic episode (ORs, 1.45-2.70) and antisocial, avoidant, schizoid, paranoid, and obsessive-compulsive personality disorders (ORs, 1.31-2.55). Compared with normal weight individuals, being moderately overweight was significantly associated with anxiety and some substance use disorders, but not mood or personality disorders. Specific anxiety disorders that occurred at significantly higher rates among all categories of persons exceeding normal weight were generalized anxiety, panic without agoraphobia, and specific phobia (ORs, 1.23-2.60). Being underweight was significantly related to only a few disorders; it was positively related to specific phobia (OR, 1.31) and manic episode (OR, 1.83), and negatively associated with social phobia (OR, 0.60), panic disorder with agoraphobia (OR, 0.40), and avoidant personality disorder (OR, 0.59). These data provide a systematic and comprehensive assessment of the association between body weight and psychiatric conditions. Interventions addressing weight loss may benefit from integrating treatment for psychiatric disorders.