In a retrospective study of 180 inpatients with DSM-III borderline personality disorder (BPD), the degree and direction of psychiatric comorbidity were used to examine the extent to which BPD is a homogeneous entity with clearly defined boundaries. Ninety-one percent of patients with BPD had one additional diagnosis, and 42% had two or more additional diagnoses. Both patients with BPD and controls with other personality disorders had similar rates and directions of comorbidity. The two groups did not differ significantly in prevalence of affective disorder. The DSM-III BPD appears to constitute a very heterogeneous category with unclear boundaries, overlapping with many different disorders but without a specific association with any one Axis I disorder. Comorbidity in patients with BPD may reflect base rates of psychopathology rather than anything inherent to BPD. Future studies should control for comorbidity to ensure homogeneity of comparison groups.