Objectives. Certain studies have claimed that borderline personality disorder (BPD) could be evaluated as a subtype of bipolar disorder (BD), whereas others have argued that BPD should be regarded as an independent disorder because of its distinct clinical features. The aim of this study was to investigate if there was a difference between these 2 disorders biologically based on EEG recordings. Methods. A total of 111 subjects (11 healthy, 25 BPD, 75 BD) who had resting EEG recordings were included. The EEGs were analyzed to compute absolute power values. Results. One-way analysis of variance results revealed statistically significant differences among the 3 groups on 55 out of 229 EEG variables. However, post hoc analysis indicated that all of the significant changes were between healthy and patient groups and no significant differences were found between 2 clinical groups. Conclusion. The findings suggested that these 2 clinical entities are biologically similar; however, further research should be performed to explain the basis clinical differences between the 2 disorders.