Twenty-three children with Langerhans' cell histiocytosis (LCH) have been treated with trimethoprim-sulphamethoxazole (T-S) in a 4-year period. The children are classified in two main groups according to the extent of their disease. Group A includes 16 children with single system disease and group B, seven children with multisystem disease. All patients were treated for 4 weeks to 3 months. The results of treatment are evaluated in terms of response in individual organs involved. All children with single system disease had a good response to the drug. Children with multisystem disease had a good response to some organs but a poorer outcome for the lungs and for the blood. These patients did not respond even to conventional chemotherapy.