There is need for a validated short, simple instrument to quantify chronic obstructive pulmonary disease (COPD) impact in routine practice to aid health status assessment and communication between patient and physician. Current health-related quality of life questionnaires provide valid assessment of COPD, but are complex, which limits routine use. The aim of the present study was to develop a short validated patient-completed questionnaire, the COPD Assessment Test (CAT), assessing the impact of COPD on health status. 21 candidate items identified through qualitative research with COPD patients were used in three prospective international studies (Europe and the USA, n = 1,503). Psychometric and Rasch analyses identified eight items fitting a unidimensional model to form the CAT. Items were tested for differential functioning between countries. Internal consistency was excellent: Cronbach's alpha = 0.88. Test re-test in stable patients (n = 53) was very good (intra-class correlation coefficient 0.8). In the sample from the USA, the correlation with the COPD-specific version of the St George's Respiratory Questionnaire was r = 0.80. The difference between stable (n = 229) and exacerbation patients (n = 67) was five units of the 40-point scale (12%; p<0.0001). The CAT is a short, simple questionnaire for assessing and monitoring COPD. It has good measurement properties, is sensitive to differences in state and should provide a valid, reliable and standardised measure of COPD health status with worldwide relevance.