OBJECTIVE: Chronic arthritis in children represents a nonhomogeneous group of diseases with unknown etiology. To classify these patients in well defined diagnostic categories, a task force of the International League Against Rheumatism proposed a new classification with precise criteria. We analyzed the new criteria in children with chronic arthritis. METHODS: A cohort of children was prospectively and sequentially examined in a pediatric rheumatology clinic from April to June 1997. RESULTS: One hundred ninety-four children fulfilled the criteria of juvenile idiopathic arthritis and 80% of them (155 children) were classified in one of the 6 diagnostic categories. Seventeen children (9%) did not fit any other category and 22 (11%) could be classified in more than one category. The proportion of children fitting only one category was much lower for psoriatic arthritis and enthesitis related arthritis than for the other categories. CONCLUSION: Based on the results, we propose some modifications to the classification criteria. This new classification is an important tool for the diagnosis of chronic arthritis in children, but the criteria need further adjustments to improve the percentage of patients classified in one defined category.