In dogs, atopic dermatitis (AD) is a common and chronic allergic skin disease that often necessitates treatment with pharmacological interventions. In the last 30 years, numerous clinical trials testing the efficacy of anti-inflammatory drugs have been reported, but there has been a lack of consistency in the assessment of outcome measures. Several clinical scales have been employed over time, but none of these scoring systems were ever tested for validity and reliability. A committee of the International Task Force on Canine Atopic Dermatitis evaluated the currently available scales used to assess disease morbidity in humans and dogs with AD, and a third version of the Canine Atopic Dermatitis Extent and Severity Index (CADESI-03) was designed. This version was expanded from previous ones by redistribution and increase in body sites tested, the use of an additional lesion reflecting underlying pruritus (e.g. self-induced alopecia) and an increase in the numerical range of severity for each lesion. The CADESI-03 scale was tested for validity and reliability in a cohort of 38 dogs with AD. Overall, this revised version of the CADESI was found to exhibit acceptable content, construct, criterion, and inter- and intra-observer reliability and sensitivity to change. As a result, this scale is recommended as a validated tool for assessment of disease severity in clinical trials testing the efficacy of interventions in dogs with AD.