The authors developed the Yale Brown Obsessive Compulsive Scale Modified for Body Dysmorphic Disorder (BDD-YBOCS), a 12-item semistructured clinician-rated instrument designed to rate severity of body dysmorphic disorder (BDD). The scale was administered to 125 subjects with BDD, and interviews with 15 subjects were rated by 3 other raters. Test-retest reliability was assessed in 30 subjects. Other scales were administered to assess convergent and discriminant validity, and sensitivity to change was evaluated in a study of fluvoxamine. Each item was frequently endorsed across a range of severity. Good interrater reliability, test-retest reliability, and internal consistency were obtained. BDD-YBOCS scores correlated with global severity scores but not with a measure of general psychopathology; they were modestly positively correlated with depression severity scores. Three factors accounted for 59.6 percent of the variance. The scale was sensitive to change in BDD severity. The BDD-YBOCS appears to be a reliable and valid measure of BDD severity and is a suitable outcome measure in treatment studies of BDD.