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A new "severity specifier" for bulimia nervosa (BN), based on the frequency of inappropriate weight compensatory behaviours (IWCBs), was added to the DSM-5 as a means of documenting heterogeneity and variability in the severity of the disorder. Yet, evidence for its validity in clinical populations, including prognostic significance for treatment outcome, is currently lacking. Existing data from 281 treatment-seeking patients with DSM-5 BN, who received the best available treatment for their disorder (manual-based cognitive behavioural therapy; CBT) in an outpatient setting, were re-analysed to examine whether these patients subgrouped based on the DSM-5 severity levels would show meaningful and consistent differences on (a) a range of clinical variables assessed at pre-treatment and (b) post-treatment abstinence from IWCBs. Results highlight that the mild, moderate, severe, and extreme severity groups were statistically distinguishable on 22 variables assessed at pre-treatment regarding eating disorder pathological features, maintenance factors of BN, associated (current) and lifetime psychopathology, social maladjustment and illness-specific functional impairment, and abstinence outcome. Mood intolerance, a maintenance factor of BN but external to eating disorder pathological features (typically addressed within CBT), emerged as the primary clinical variable distinguishing the severity groups showing a differential treatment response. Overall, the findings speak to the concurrent and predictive validity of the new DSM-5 severity criterion for BN and are important because a common benchmark informing patients, clinicians, and researchers about severity of the disorder and allowing severity fluctuation and patient's progress to be tracked does not exist so far. Implications for future research are outlined.