Diagnostic criteria for borderline personality disorder (BPD) include instability in identity and interpersonal relationships. Here, we probed whether instability is already present in BPD patients' thoughts about themselves and others. We tested BPD patients (N=27) and healthy controls (N=25) with a mind-wandering task that assesses content and variability of stimulus-independent self-generated thoughts. Multi-level modeling revealed that while BPD patients and healthy controls mind-wander to a similar extent, BPD patients' thoughts are colored predominantly negatively. Most importantly, although their thoughts concerned the self and others as much as in controls, they fluctuated more strongly in the degree to which their thoughts concerned themselves and others and also gave more extreme ratings. Self- and other related thoughts that were more extreme were also more negative in valence. The increased variability supports current conceptualizations of BPD and may account for the instability in identity and interpersonal relationships.