Summary Background Patients with psoriasis have significant impairment in their quality of life (QoL). Several patient-completed instruments are available to measure outcome in dermatological conditions but these primarily focus on severity of disease rather than on QoL. Objectives To develop a new instrument specifically designed to measure QoL in psoriasis—the Psoriasis Index of Quality of Life (PSORIQoL). Methods The content of the instrument was derived from qualitative interviews with 62 psoriasis patients in the U.K., Italy and the Netherlands. Content analysis of the interview transcripts from the three countries identified potential items for the questionnaire. All further stages of the project were conducted in the U.K. only. Face and content validity were determined by means of a focus group held with seven psoriasis patients and face-to-face interviews with a further 21 patients. A new draft version of the PSORIQoL, together with the Dermatology Life Quality Index (DLQI) and the General Well-Being Index (GWBI), were completed by a new sample of psoriasis patients contacted by mail, in order to determine construct validity. A subsample of the respondents completed a similar package 2 weeks later, to assess reproducibility (reliability). Results The first version of the PSORIQoL consisted of 61 items covering the needs identified in the interviews as having been affected by psoriasis or its treatment. These included fear of negative reactions from others, self-consciousness and poor self-confidence, problems with socialization, physical contact and intimacy, limitations on personal freedom and impaired relaxation, sleep and emotional stability. Following the focus group and field-test interviews, a new draft measure consisting of 45 items was completed by 148 patients, 88 of whom completed and returned a similar package 2 weeks later. Application of Rasch analysis identified a final 25-item version of the PSORIQoL. This version had a test–retest reliability coefficient of 0·89, indicating good reproducibility. The levels of association with the DLQI and PGWB indicated that the PSORIQoL measures a related but distinct construct. Conclusions The PSORIQoL appears to be a practical, reliable and valid instrument for measuring the impact of psoriasis on QoL. It remains necessary to establish the instrument's responsiveness to changes in QoL associated with treatment.