Identity diffusion is a core element of the borderline personality organization. A valid and reliable assessment tool is needed to identify at-risk adolescents in the Italian context. In this study, we investigated the psychometric properties of the Assessment of Identity Development in Adolescence (AIDA), designed to assess identity diffusion vs. identity integration, in an Italian sample (N = 1,102) of clinical and nonclinical adolescents. Explorative structural equation modelling fit the expected bi-factor structure with one pathology-related general factor and 6 specific factors (CFI = .905, RMSEA = 0.036). Internal consistency Cronbach's alphas were high with .94 for the AIDA total scale and adequate for the six subscales with .68 to .81. Results supported convergent validity with measures of borderline personality features (BPFSC-11), as well as discriminant validity with identity dimensions in ideological and interpersonal domains (UMICS). The AIDA total score Identity Diffusion differed with significance p < .001 and with a large effect size of d = 1.5 standard deviations between the general population sample and N= 55 patients diagnosed with Borderline Personality Disorder. AIDA is a valid and reliable measure to assess adolescents' identity pathology within the Italian context.