Ms. A, a 36-year-old woman, had pronounced repetitive self-mutilating behavior in addition to borderline personality disorder diagnosed according to an interview with the German version of the Dissociative Disorders Interview Schedule +(1) (DSM-IV diagnoses 296.32, 300.6, 300.12, 300.15, 300.81, 301.83, and 307.1). She collected her entire nocturnal (8:00 p.m. to 8:00 a.m.) urine output on 86 consecutive nights +(2) Her nocturnal urinary volumes were almost identical (mean=983 ml, SD=128), but the fluctuations in her nocturnal cortisol excretion were extreme, varying from 2 to 30 μg a night. Compared to normative data (cortisol excretion during 24 hours: 20–90 μg/day), her nocturnal cortisol secretion appears low on average; this is in accordance with data from other studies +(3). However, our longitudinal study revealed that periods of low cortisol excretion were followed by periods of continuous increases in excretion over several nights. Above 20 μg a night, she performed one or several acts of self-mutilation. Subsequently, an instantaneous decrease to low initial baseline values of cortisol was observed. Thereafter, her nocturnal cortisol excretion remained at this low level for several days. The next period of increasing cortisol secretion was again terminated by an episode of self-mutilation.