This study examined the psychometric properties of the Defense Style Questionnaire (DSQ-40) in adolescents. Internal consistency, factor structure, and discriminant and concurrent validity of the DSQ-40 were studied in 211 adolescent psychiatric outpatients aged 13 to 19 years and 199 age-matched and sex-matched controls. Principal components analysis yielded four internally consistent components: mature, neurotic, image-distorting, and immature defense styles. The outpatients reported more immature, image-distorting, and neurotic styles and less mature style than did the controls, suggesting adequate discriminant validity. As a demonstration of convergent and concurrent validity, the severity of psychiatric symptoms assessed by the General Health Questionnaire and psychosocial adjustment assessed by the Global Assessment of Functioning Scale correlated theoretically meaningfully with the different defense styles. The DSQ-40 appears to be a reliable and valid instrument for adolescents.