AbstractObjectiveImpairments in personality functioning (PF) according to the Alternative Model of Personality Disorders (AMPD) may affect anorexia nervosa (AN) and its treatment. However, PF impairments in adolescents with AN have not been studied. This case‐control study analysed PF impairments in adolescent inpatients suffering from AN compared to a clinical control group (CC) and investigated the relationship of PF with AN severity.MethodAdolescent patients with AN (n = 43) and CC (n = 127) were matched by age, sex, and global functioning. Validated interviews were applied to assess impairments in PF (Semi‐Structured Interview for Personality Functioning, STiP‐5.1) and severity of AN (Eating Disorder Examination, EDE), as well as weight measures. Group comparisons of PF impairment and the relationship between PF impairment and AN severity were analysed using multiple linear regression.ResultsAN patients showed greater impairment in the facet ‘self‐esteem’ (p = 0.033, Cohen's f² = 0.036) than CC. In AN patients, impairments in the domain ‘self‐functioning’ (p = 0.003, Cohen's f² = 0.254) and its underlying element ‘identity’ (p = 0.016, Cohen's f² = 0.172) were associated with AN severity (EDE). Additionally, higher impairments in two facets of ‘empathy’ (p = 0.019, Cohen's f² = 0.223) and ‘intimacy’ (p = 0.026, Cohen's f² = 0.199) were associated with higher AN severity (lower BMI percentiles).ConclusionsThese results support theoretical frameworks of AN and highlight the importance of PF impairments in understanding and treating AN. Future research should investigate how impairments in PF affect AN development and treatment response through longitudinal studies.