Nonsuicidal Self-Injury (NSSI) is prominent in individuals with Borderline Personality Disorder (BPD), and there is abundant evidence that affect regulation plays an important role in NSSI in the majority of patients. Affective variability is a core feature of BPD, and thus, we hypothesize that NSSI has an affect stabilization function in BPD. Affect stabilization is a process through which individuals attempt to make their affect more stable by reducing affective variability. We tested this hypothesis in 32 participants with BPD who reported on their NSSI and affect -using a displeasure-pleasure (valence) and activation deactivation (activation) dimension in an experience sampling study with 10 random signals scheduled per day for 8 days. Results indicated that individuals who engaged in NSSI show more Within Subject (WS) variance in valence and activation than individuals who did not engage in NSSI. However, within the NSSI patients, individuals who engaged more frequently in NSSI during the study showed less WS variance in valence and activation than patients who engaged less frequently in NSSI. This suggests that NSSI may be reinforced by its affect stabilization function. In the discussion, we explore alternative explanations for the relation between NSSI and affective variability, and consider the clinical implications. (C) 2017 Elsevier Ltd. All rights reserved.