To validate the European League Against Rheumatism (EULAR), the American College of Rheumatology (ACR), and the World Health Organization (WHO)/International League Against Rheumatism (ILAR) response criteria for rheumatoid arthritis (RA). EULAR response criteria were developed combining change from baseline and level of disease activity attained during follow up. In a trial comparing hydroxychloroquine and sulfasalazine, we studied construct (radiographic progression), criterion (functional capacity), and discriminant validity. EULAR response criteria had good construct, criterion, and discriminant validity, ACR and WHO/ILAR criteria showed only good criterion validity. EULAR response criteria showed better construct and discriminant validity than did the ACR and the WHO/ILAR response criteria for RA.