The Dermatology Life Quality Index (DLQI) is one of the most frequently used questionnaires to evaluate the impact of dermatological diseases on patients' lives. This study aimed to assess the reliability and validity of the instrument and to test its unidimensionality in a large sample of patients with psoriasis (n=976) hospitalized at IDI-IRCCS, Rome, Italy. Nine hundred patients completed the DLQI, the Psoriasis Disability Index (PDI) and the Skindex-29 (response rate 92%). The internal consistency of the DLQI was high (Cronbach's alpha=0.83). Evidence of convergent validity was provided by high (r=0.64-0.81) correlations between the DLQI, the PDI, and the functioning and emotions scales of Skindex-29. Exploratory factor analysis indicated the presence of four different principal common factors. Confirmatory factor analysis showed a clear second-order factor structure, with a homogeneous second-order factor underlying the four primary-surface factors. This study confirms that the DLQI is a reliable and valid instrument to assess patient-perceived impact of skin disease. Also, it supports the unidimensionality of the DLQI and hence corroborates the common practice of using the total score.