OBJECTIVES: To create a new instrument to evaluate quality of life (QoL) in patients with psoriasis, from five preexistent instruments, using Rasch analysis. METHODS: A Rasch analysis was performed on five QoL instruments, in a group of 936 patients with psoriasis, recruited in a dermatological hospital. Two instruments were dermatology-specific [Skindex-29 and Dermatology Life Quality Index (DLQI)], and three were psoriasis-specific [Psoriasis Disability Index (PDI), Psoriasis Life Stress Inventory (PLSI), and Impact of Psoriasis questionnaire (IPSO)]. The total number of items was 85. The number of categories was reduced to three in each instrument on the basis of threshold probability curves. Having identified threshold and item performance for each scale separately, items from all five scales were analyzed together. Items were eliminated on the basis of poor fit to the model, dependencies amongst items, item location and qualitative meaning of the items, with the overall goals of obtaining a uniform and wide spread of the items as possible, and a balance among the different subscales. RESULTS: We obtained a final questionnaire of 30 items: 4 symptom, 9 emotion, 10 functioning, and 7 social items. Fifteen out of 29 items came from the Skindex-29, 8 from the PLSI, 3 from the IPSO, 2 from the DLQI, and 2 from the PDI. The fit of these items to the Rasch model was satisfactory. The final Person Separation Index (PSI) was 0.916. CONCLUSIONS: The advantages of this new instrument are that data for it fit the Rasch model, and that it summarizes information from five questionnaires (i.e., 85 items) with just 30 items. Moreover, the 30-item questionnaire is working better than the five instruments together, as the properties we analyzed show. The new instrument will be validated on a new sample of 300 patients with psoriasis.