Ego defense mechanisms were compared in adolescent psychiatric inpatients and healthy adolescents to determine their relationship to specific diagnoses and to negative emotions. Seventy-one patients with schizophrenia, 28 with major depressive disorder, and 24 with obsessive- compulsive disorder (OCD) and 87 normal adolescents were assessed for defense mechanisms by the Life Style Index (LSI) and Ego Defense Scale (EDS). The Beck Depression Inventory (BDI), State-Trait Anxiety Inventory (STAI), and Multidimensional Anger Inventory (MAI) were used to assess depression, anxiety, and anger. Several defenses distinguished all psychiatric patients from controls, and a few defenses characterized different patient groups, especially those with OCD. Projection (on the LSI), displacement, and regression correlated significantly with anger; displacement, reaction formation, and undoing on the EDS correlated with anxiety, and denial was negatively correlated with depression. These findings have some relevance for the evaluation of Plutchik’s psychoevolutionary theory of emotions and for the further empirical study of defenses in psychopathology.