The relationship between borderline personality disorders (BPD) and major depressive disorder (MDD) continues to be controversial. A reliable biological marker for depressed BPD patients would not only support the diagnosis but could also help in predicting treatment outcome. A large sample of psychiatric patients was screened and data on the Dexamethasone Suppression Test (DST) were obtained for 67 patients who met the criteria for BPD by scoring 7 or greater on the Diagnostic Interview for Borderlines. The DST was positive in 23.9% of the cases. Fifty cases of BPD also met the Research Diagnostic Criteria for MDD. The DST was positive in 26.0%. Of the 50 patients with MDD, 34 also met the criteria for endogenous depression. Only 17.6% of this subgroup had positive results on the DST. The low sensitivity and specificity of the DST for depression in BPD patients suggests that the DST is not a useful test in differentiating BPD patients with MDD from those without MDD. The possible reasons for the DST not being useful in this population are discussed. These findings raises further questions about the nature of the depression suffered by BPD patients.