Objectives This research evaluated the psychometric properties of a new Psoriasis Symptom Diary, identified diary responder definitions for use in determining whether a patient has experienced clinically meaningful change, and refined diary item content for use in future clinical trials. Methods The Psoriasis Symptom Diary was administered in a phase 2 clinical trial of AIN457 to US adult outpatients (N = 172) with physician-diagnosed moderate to severe chronic plaque-type psoriasis. Participant compliance with daily diary administration and item score variability, reliability, construct and discriminant validity, sensitivity to change, and interpretation were all evaluated. Results Participants completed 94% of scheduled diary assessments across 12 study weeks. Diary items were generally normally distributed, and no floor or ceiling effects were observed. Item reliability (reproducibility) was acceptable (intraclass correlation coefficients > 0.80), with an exception for one item (skin color). At week 12, items significantly related to criterion measures as predicted (Psoriasis Area and Severity Index r = 0.27-0.57; Investigator's Global Assessment r = 0.25-0.59), with the exception of items that measured skin color and difficulty using hands. Most items generated change scores that were synchronous to changes as measured by the Psoriasis Area and Severity Index, Investigator's Global Assessment, Dermatology Life Quality Index (r > 0.37), as well as the Patient Global Impression of Change. Responders experienced a 2- to 3-point and 3- to 5-point change in item scores for minimal and large improvements, respectively. Four items that did not perform well were dropped from the diary. Conclusions The 16-item Psoriasis Symptom Diary demonstrated favorable psychometric properties and is a brief, useful tool for measuring patient-based symptoms and the impact of chronic plaque psoriasis. © 2013 International Society for Pharmacoeconomics and Outcomes Research (ISPOR).