GM-CSF induces proliferation and activation of Langerhans' cells in vitro. The density of Langerhans' cells in human tumours is correlated to the in situ density of GM-CSF, and intradermal injection of GM-CSF induces local accumulation of Langerhans' cells. Therefore, we investigated the presence of GM-CSF in the sera of children with Langerhans' cell histiocytosis (LCH). We detected GM-CSF in the sera of all children with disseminated and active LCH, but not in the sera of patients with localized (i.e. bone) LCH. These results suggest that GM-CSF level is related to extent and the activity of LCH.