L1 is a major granulocyte and monocyte protein with Mr 36.5 kDa. It is released during leukocyte activation and detected in plasma by use of an enzyme immunoassay. In our study, L1, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), orosomucoid and haptoglobin were analyzed in 127 patients with juvenile rheumatoid arthritis (79 pauci, 33 poly and 15 systemic). L1, ESR and the acute phase proteins were all found to have positive correlations with the clinical joint assessments, with L1 having the strongest correlations. High correlation coefficients were found between L1 and orosomucoid (r = 0.83), ESR (r = 0.72), haptoglobin (r = 0.71) and CRP (r = 0.65), with p less than 0.001 for all the correlations.