The purpose of this study was to assess the prevalence of axis I disorders among patients with borderline personality disorder over 6 years of prospective follow-up. A semistructured interview of demonstrated reliability was used to assess presence or absence of comorbid axis I disorders in 290 patients who met Revised Diagnostic Interview for Borderlines criteria and DSM-III-R criteria for borderline personality disorder and 72 patients who did not meet these criteria but did meet DSM-III-R criteria for another axis II disorder. Over 94% of surviving patients were reinterviewed about their axis I disorders at 2-year, 4-year, and 6-year follow-up periods. Although the patients with borderline personality disorder experienced declining rates of many axis I disorders over time, the rates of these disorders remained high, particularly the rates of mood and anxiety disorders. Patients whose borderline personality disorder remitted over time experienced substantial decline in all comorbid disorders assessed, but those whose borderline personality disorder did not remit over time reported stable rates of comorbid disorders. When the absence of comorbid axis I disorders was used to predict time to remission, the absence of substance use disorders was a far stronger predictor of remission from borderline personality disorder than was the absence of posttraumatic stress disorder, mood disorders, other anxiety disorders, or eating disorders, respectively. The results of this study suggest that axis I disorders are less common over time in patients with initially severe borderline personality disorder, particularly for patients whose borderline personality disorder remits over time. The findings also suggest that substance use disorders are most closely associated with the failure to achieve remission from borderline personality disorder.