Bipolar disorder (BD) and borderline personality disorder (BPD) are disabling and life-threatening conditions. Both disorders share relevant comorbidities, particularly the risk of having a lifetime substance use disorder (SUD). We tested the hypothesis that patients with both BD type I (BDI) or II (BDII) and BPD would have a higher rate of SUD than would patients with either disorder alone. A total of 3651 psychiatric patients were evaluated with semistructured diagnostic interviews for Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, axis I and II disorders. A total of 63 patients were diagnosed with both BD and BPD, and these patients were significantly more likely to have a SUD compared with BDII patients without BPD (76% vs. 50%, χ2 = 9.69, p < 0.01). There were no differences when comparing the comorbid group with BPD patients without BD (76% vs. 71%, χ2 = 0.519, p = 0.4). The present study shows the importance of taking both BPD and BD into consideration insofar as the co-occurrence of the disorders increased the risk of having a SUD especially when compared with BDII alone.