The Urticaria Activity Score (UAS) is a widely used patient-reported outcome measure for patients with chronic idiopathic urticaria (CIU) that includes 2 items: intensity of pruritus and number of hives. Items are scored individually, and the UAS7 is calculated as the sum of pruritus and number of hives over 1 week. Recently, its instructions were enhanced. To assess the measurement properties of the enhanced UAS. Seventy-three subjects with CIU completed the UAS with enhanced instructions, other measures of disease activity including the size of the largest hive, and collateral measures during a multicenter, randomized, double-blind, placebo-controlled study of omalizumab for the treatment of CIU. The minimal important difference (MID) was estimated through distribution- and anchor-based approaches. Test-retest reliability was assessed with the intraclass correlation coefficient (ICC); internal consistency reliability was evaluated with Cronbach's alpha; 3 responsiveness coefficients were calculated; known groups validity was assessed based on physician in-clinic UAS scores; and construct validity was assessed through Spearman correlation coefficients with collateral measures. The MID ranged from 9.5 to 10.5 for the UAS7, 5.0 to 5.5 for number of hives (weekly average), and 4.5 to 5.0 for pruritus and size of largest hive (weekly average). Internal consistency was supported by alpha coefficients greater than 0.80. The ICC values for test-retest reliability ranged from 0.602 to 0.884. For subjects on active treatment, responsiveness coefficients were greater than 0.80. Known-groups validity was supported for most UAS scores; and construct validity was demonstrated by relationships with collateral measures. The enhanced UAS has adequate measurement properties to support its use in clinical research.