OBJECTIVE: To evaluate the diagnostic accuracy of the 2010 American College of Rheumatology/European League Against Rheumatism (ACR/EULAR) and 1987 ACR criteria for rheumatoid arthritis (RA), and the respective role of the algorithm and scoring of the ACR/EULAR. METHODS: In total, 270 patients with recent-onset arthritis of < 1 year duration were included prospectively between 1995 and 1997 and followed for 2 years. RA was defined as the combination, at completion of followup, of RA diagnosed by an office-based rheumatologist and treatment with a disease-modifying antirheumatic drug or glucocorticoid. We compared the sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of the criteria sets in the overall population, in the subgroup meeting the tree condition for ACR/EULAR scoring, and in the overall population classified according the full tree. RESULTS: At baseline, 111 of the 270 patients had better alternative diagnoses and 16 had erosions typical for RA; of the 143 remaining patients, 52 had more than 6 ACR/EULAR 2010 points (indicating definite RA) and 91 had fewer than 6 points. After 2 years, 11/16 patients with erosions and 40/52 with more than 6 points had RA. 100 of the 270 patients met the reference standard for RA. Sensitivity, specificity, PPV, and NPV of the ACR/EULAR (full tree) were 51/100 (51%), 153/170 (90%), 51/68 (75.4%), and 153/202 (75.7%), respectively. Diagnostic accuracies of the ACR/EULAR score and ACR 1987 criteria were not statistically different. CONCLUSION: Much of the improvement of the ACR/EULAR criteria was ascribable to the use of exclusion criteria in the algorithm.