objective:to analyze the relationship between the acute phase reactants and the disease activity of juvenile idiopathic arthritis (jia) and to evaluate the agreement between erythrocyte sedimentation rate and c-reactive protein during the acute phase of the disease. methods: a cohort retrospective study has been conducted based on the analysis of 30 children and adolescents who fulfilled the diagnostic criteria of jia. all of them were in current follow-up at the pediatric rheumatology outpatient clinic and had acute phase reactants blood tests performed. results: studied population comprised 30 patients: 21 (70%) of them were females and 19 (63.3%) presented oligoarticular subtype. the mean age at disease onset was 65.6 months; the age at diagnosis was 85.3 months and the follow-up had 57.2 months of duration. the acute phase reactants showed positive association with the disease activity. anemia was not associated with disease activity. during the acute phase of the disease, agreement between erythrocyte sedimentation rate and c-reactive protein was greater than 80%. conclusions: the acute phase reactants have a positive association with the activity of the disease and using both tests simultaneously increases their specificity.