Borderline personality disorder (BPD) is characterized by rapidly shifting symptoms, including intense anger and aggressive behavior. Understanding how fluctuations in ovarian hormones across the menstrual cycle may contribute to symptom instability is key for accurate assessment of BPD symptoms and effective interventions. Reactive and proactive aggression, as well as anger-in and anger-out, were assessed daily in 15 physically healthy, unmedicated naturally cycling female individuals meeting criteria for BPD across 35 days. Urine luteinizing hormone surge and salivary progesterone were used to confirm ovulation and verify the cycle phase. Multilevel models evaluated cyclical differences of symptoms between cycle phases. Both forms of aggressive behavior demonstrated marked cycle effects, with reactive aggression highest during the midluteal cycle phase, co-occurring with initial increases in anger and irritability and followed by perimenstrual peaks in anger and anger-in. In contrast, highest levels of proactive aggression were observed during the follicular and ovulatory phases, when emotional symptoms and anger were otherwise at lowest levels. These findings highlight the importance of identifying the function of aggression when considering potential psychological and biological influences. Naturally cycling individuals with BPD may be at elevated risk for luteal worsening of a range of interpersonally reactive symptoms, including reactive aggression, whereas proactive aggression may occur more in phases characterized by less emotional and cognitive vulnerability and greater reward sensitivity. Research on aggression in this population should consider cycle effects. Cycling individuals with BPD attempting to reduce aggressive behavior may benefit from cycle-tracking to increase awareness of these effects and to develop appropriate strategies.