Vulnerability factors such as insecure attachment may have a lasting effect on the outcome of couples therapy, even long after discharge from treatment. Given that attachment has never been examined as an outcome predictor for couples therapy in the long term, the authors studied its effect on outcome during and after couples therapy. This prospective study included 71 inpatients participating in group couples therapy who the authors measured at baseline, immediately posttreatment at 2 months, and at 8 and 20 months, regarding two outcomes: problem-solving capacity (using the Interactional Problem Solving Questionnaire) and psychopathology (using the 90-item Symptom Check List). At baseline, the authors measured partner attachment (using the Experiences in Close Relationships Questionnaire). Mixed model analyses showed that attachment-related dysfunctional working models of self and others predicted less improvement in psychopathology (p = .04) and problem-solving capacity (p = .01), respectively. Special attention to insecure attachment in couples therapy may therefore prove valuable in terms of outcome in the long run.