The Brown Assessment of Beliefs Scale (BABS) is a widely used measure that assesses insight/delusionality—an important dimension of psychopathology—both dimensionally and categorically (e.g., delusional versus nondelusional). The BABS has been shown to have good psychometric properties in a number of disorders, but sample sizes were small. In the present study, 327 subjects with body dysmorphic disorder (BDD) were interviewed with the BABS to assess insight regarding appearance beliefs. Other measures were administered. Intraclass correlation coefficients demonstrated excellent interrater reliability and test-retest reliability; internal consistency was strong. Principal components factor analysis identified one factor accounting for 60% of the variance. Analyses with measures of severity of BDD, depressive symptoms, and general psychopathology indicated good discriminant validity. Among the treated subjects, the BABS was sensitive to change but not identical to improvement in symptom severity. These findings provide further evidence that the BABS is a reliable and valid measure of insight/delusionality.