This study represents an attempt to collect observations from co-operating countries to evaluate the previously suggested criteria for juvenile chronic arthritis during the onset periods 0-3 and 3-6 months, preferably studied prospectively. Only 267 of 378 forms returned were satisfactory for inclusion because of failure to observe exclusion criteria or insufficient information. Despite this it was possible to conclude that systemic disease represented 25% of the group and that these features tended to decline during the 6 month period. Gut inflammation was seen in 4.5% and the frequency of chronic iridocyclitis increased during the observation period to 4.5%. Variation in joint state during this period suggests that course classification may be important, as 23 of 118 who were pauciarticular at 3 months became polyarticular by six months, but only 46 of 76 who were polyarticular at 3 months remained in this state at six months.