The objective of this study is to identify baseline factors associated with rheumatoid arthritis (RA) diagnosis at the end of 1-year follow-up in a cohort of patients with very recent onset arthritis. Incident cases with self-reported arthritis (<or=12 weeks) referred by primary care physicians were assessed by a designated rheumatologist who predicted in those with >or=1 swollen joint the diagnosis of RA at the end of follow-up. Patients were regularly seen and diagnosed through follow-up by staff rheumatologists who were blind to diagnostic prediction. Of 119 referrals, 78 (65.5%; age 35.5 +/- 13.5 years; 69 females) were diagnosed at baseline as very recent onset arthritis (median duration 6 weeks (0-12 weeks)); of 75 patients completing 1-year follow-up, 51 (66.5%) were classified as RA; 12 (16%) had self-limited arthritis; and 13 (17.5%) other diagnoses. The characteristics of patients with RA as final diagnosis were polyarthritis, morning stiffness >or=1 h, high counts of swollen joints, and low frequency of systemic symptoms. Rheumatologist prediction of RA and anti-cyclic citrullinated peptide (anti-CCP) antibodies was strongly associated with RA as a final diagnosis in the logistic regression analysis. Sensitivity and specificity of the rheumatologist prediction were 94% and 74%, for anti-CCP antibodies, 56% and 96%; the combination of both variables had a specificity of 100% and a sensitivity of 53%, and a positive predictive value of 98%. The combination of RA as predicted diagnosis by a rheumatologist and anti-CCP antibodies is highly specific for RA diagnosis in patients with very early arthritis.