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 diagnosis 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 the use 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 short-term 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.