The investigation of comorbidity between major depressive disorder (MDD) and personality disorders (PDs) has attracted considerable interest. Whereas some studies found that the presence of PDs has adverse effects on the course and treatment of MDD, others have failed to demonstrate this link. These inconsistent findings suggest that specific PD comorbidity might affect the course of MDD by modulating factors that increase the overall risk of depression, including an elevated tendency to perceive stress. To investigate whether the presence of a specific PD cluster was associated with elevated levels of stress appraisal, we administered the Perceived Stress Scale (PSS) before and after treatment to 227 MDD outpatients enrolled in an 8-week open-label treatment with fluoxetine. Following treatment, multiple linear regression analyses revealed that the presence of Cluster A, but not Cluster B or C, was associated with higher levels of perceived stress, even after adjusting for baseline depression severity and PSS scores, as well as various sociodemographic variables. The presence of Cluster A PD comorbidity was uniquely associated with elevated stress appraisal after antidepressant treatment, raising the possibility that stress exacerbation might be an important factor linked to poor treatment outcome in MDD subjects with Cluster A pathology.