This work aims to further validate the object-relations–based model of personality pathology assessment, evaluating the psychometric properties of the Italian version of the Inventory of Personality Organization (IPO), a self-report instrument for the assessment of personality organization according to O. Kernberg's model of personality pathology. Six hundred ninety-six nonclinical volunteers and 121 psychiatric patients completed a set of questionnaires including the IPO, the Severity Indices of Personality Problems, the Borderline Personality Disorder Checklist, the Response Evaluation Measure 71, and the Symptom Checklist 90–Revised. Confirmatory factor-analyses on the IPO items supported the 1-, 2-, 3-, and 4-factor solutions. The last (Instability of sense of self/others, Instability of goals, Instability of behaviors, Psychosis) resulted in relatively better fit indexes. Invariance across samples (nonclinical, clinical) and gender was confirmed. The 4 IPO subscales showed good levels of internal coherence and, in the nonclinical sample, good test–retest reliability. Associations with the convergent measures were in line with theoretical expectations and supported the benefit of adopting a 4-factor solution. The 4 factors showed the expected criterion relations: All the dimensions discriminated between clinical and nonclinical subjects, whereas only Instability of self/others and Instability of goals discriminated patients with borderline personality disorder from patients with other diagnoses. Our results suggest that the Italian version of the IPO is a reliable and valid tool for the assessment of personality organization according to Kernberg's model. Results are discussed in the context of the current directions in the evaluation of personality disorders proposed by the Diagnostic and Statistical Manual of Mental Disorders, 5th edition.