We assessed the sensitivity and specificity of the ACR 1987 revised criteria for rheumatoid arthritis (RA) in 121 patients with recent-onset (< or = 6 months) RA, 68 with reactive arthritis (ReA), 19 with ankylosing spondylitis (AS), and 13 with psoriatic arthritis (PsA). The sensitivity of each single criterion ranged 8.3-90.9% and specificity 52.0-100%. The sensitivity of four fulfilled criteria was 83.5% and specificity 86.0%. In ReA 11.8%, AS 5.3%, and PsA 38.5% of patients fulfilled four criteria respectively. Thus at the beginning of RA, 83% of patients could be diagnosed correctly by using the ACR 1987 criteria, and the remaining 17% had seropositive and/or erosive arthritis at the onset. The suitability of the radiographic ACR criteria is discussed.