Thirty five children with ambulant JCA were studied to assess the biochemical parameters of bone metabolism. The mean age of the study group was 8.8 +/- 4.1 years and the mean duration of active disease 3.8 +/- 1.3 years. According to the onset of the disease the children belonged to the systemic (7), polyarticular (12) and pauciarticular type (16). All the patients were treated with NSAIDs. In addition the polyarticular group received either gold injections or D-penicillamine and the systemic group, steroids for at least 3 months. Two groups of controls were studied. The first one included fifteen children without chronic arthritis or bone disease and the second, four children who were treated with corticosteroids for a variety of reasons. In the group with systemic JCA Se Pi (1.28 +/- 0.29 mmol/l) and renal phosphate reabsorption (TmP/GFR = 1.07 +/- 0.18) were significantly lower than in the control groups (1.50 +/- 0.19; 1.54 +/- 0.25 mmol/l, p < 0.01 and 1.35 +/- 0.18; 1.29 +/- 0.23 mmol/l GF, p < 0.05). Also lower were serum alkaline phosphatase (58 +/- 16.4 versus 83 /- 24.2 and 80 +/- 15.6 IU/l, p < 0.05), osteocalcin (5.5 +/- 4.7 versus 11.0 +/- 4.5 and 10.0 +/- 5.7 ng/ml, p < 0.05), 25OHD (15.6 +/- 4.9 versus 27.3 +/- 6.2 and 20.6 +/- 9.8 ng/ml, p < 0.001) and 1,25(OH)2D (12.1 +/- 6.0 versus 20.9 +/- 11.0 and 27.6 +/- 3.2 pg/ml, p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)