In this study, we evaluated the impact of defense style (DS) on outcome and its relation to the therapeutic alliance. Women with postpartum depression were allocated to a brief psychotherapeutic intervention. To evaluate DS and the therapeutic alliance, the Defense Style Questionnaire and the Working Alliance Inventory were employed. The main outcome was the Edinburgh Postnatal Depression Scale score at end point; anxiety and retention in treatment were also evaluated. Fifty-nine patients were included; 46 completed the therapy and 65.4% responded (Edinburgh Postnatal Depression Scale score <10). Intense use of immature defenses was related to persistence of depression and anxiety symptoms at end point, even when controlled for potential confounders. Results tended to confirm a hierarchy of DSs independently of the alliance. Clinicians should be aware of possible differential responses to brief psychotherapies related to DS.