We measured serum M-CSF con­centrations in 25 patients with idio-pathic thrombocytopenic purpura (ITP) to determine if they were elevat­ed in this condition, and to investigate the effect of corticosteroid therapy on the level of this growth factor in 8 ITP patients with profound thrombocyto-penia. M-CSF levels were measured by enzyme immunosorbent assay (ELISA). The mean M-CSF was sig­nificantly increased (P<0.05) in pa­tients with ITP in comparison to con­trol group (mean + SO were 929 + 127u/ml and 364 + 69 u/ml respec­tively). There was a significant negative correlation between M-CSF and plate- let count (P<0.05). The mean level of M-CSF was significantly decreased three weeks after corticosteroid thera­py in comparison to the high pretreat-ment level (P<0.05). Thus we con­cluded that the elevated level of M-CSF in children with ITP may reflect activation of reticuloendothelial sys­tem which could result in positive feedback to increase the destruction of platelets. Corticosteroid therapy could induce thrombocytosis by de­crease of M-CSF level in patients with ITP.