A study was performed of consecutive patients presenting to a Dutch early arthritis clinic with a primary suggested diagnosis of reactive arthritis due to streptococci between April 1998 and January 2003, in a well-defined reference population consisting of 600 000 inhabitants. At 1 year after presentation out of 45 acute arthritis patients with initially an elevated antistreptolysin-O and without an alternative rheumatic diagnosis only 9 patients (20%) were not diagnosed as PSRA; 16 cases (36%) were due to NGAS, 20 cases (44%) due to GAS. The estimate of the annual incidence rate of PSRA in the Netherlands during the study was 1.26 per 100 000: 0.70 GAS-related. A diagnostic set of criteria was formulated based on the original Ayoub&Ahmed criteria by adding a serological criterium ASO/antiDNaseB ratio <1.4 and excluding a clinical criterium on chronicity/recurrency of arthritis: likelihood ratio for a positive test 7.9 [95% confidence interval (95%CI: 2.7-22.7)], for a negative test 0.06 [95%CI: 0.009-0.39].