This study aimed to determine whether patterns of ego defense change.with short-term treatment of psychiatric illness. The subjects were 37 inpatients and outpatients with a DSM-III- R dragnosis of major depressive disorder being treated using standard clinical methods. Ego defenses before and 7 to 9 weeks after commencement of treatment were measured using a shortened version of the Defense Style Questionnaire. There was a significant decline in thejise of immature defenses with symptomatic recovery, but (no change in the neurotic or mature defenses. Patients with additional axis I diagnoses and/or abnormal personality traits (N=15) used more neurotic defenses than their counterparts with major depression alone (N=22), but this pattern did not change with time. The study demonstrates the shortterm mutability of immature defenses in relation to an episode of psychiatric illness and provides empirical support for the concept of temporary regression in the context of psychiatric illness episodes.