BACKGROUND: Selected personality features may represent important predisposing as well as perpetuating factors for insomnia, and previous studies stressed the importance to assess personality disorders in insomnia patients. METHODS: In order to evaluate the relationships between DSM-IV Axis II/DSM-5 Section II Personality Disorders (PDs) and insomnia, a sample of 171 consecutively admitted insomnia patients and a sample of 171 psychotherapy patients, matched on age and gender were administered the Italian translation of the Structured Clinical Interview for DSM-IV Axis II Personality Disorders, Version 2.0 (SCID-II). Among insomnia patients, 52.0% (n = 89) received at least one DSM-IV Axis II/DSM-5 Section II PD diagnosis according to SCID-II assessment. RESULTS: Any PD base rate estimate in our insomnia patient sample was significantly and markedly higher than the median and mean base rate estimates for any PD in the general population. Within-group analyses showed that Narcissistic, Not otherwise specified PD, Histrionic PD, and Borderline PD represented the most frequently diagnosed-both dimensionally and categorically-DSM-IV Axis II/DSM-5 Section II PD features in our insomnia patient sample. When continuously-scored PDs were considered, insomnia patients showed a significantly lower number of Paranoid and Borderline PD features than psychotherapy patients; however, the corresponding effect size estimates suggested that these differences were modest. None of the categorically-scored PDs significantly differentiated insomnia patients from psychotherapy patients. CONCLUSIONS: As a whole, our findings seemed to suggest that personality dysfunction may play a role in insomnia, while stressing the need for a dimensional approach to the assessment of maladaptive personality traits even in insomnia patients. Copyright © 2018 Elsevier Inc. All rights reserved.