Oral mucous membrane lesions were studied in 54 children below 12 yr of age treated with allogeneic bone marrow transplantation mainly because of hematological malignancies. Sixty-two percent of the children exhibited a wide range of oral side effects during therapy. Lesions observed during the first 2 wk prior to engraftment of the donor marrow were related to the chemo- and radiotherapy given. Oral ulcerations were seen in 34% of the children. Children given methotrexate as graft-versus-host disease (GVHD) prophylaxis exhibited oral ulcerations significantly (P less than 0.05) more often than those given cyclosporin. Oral lesions related to acute GVHD were only observed in two patients. Reactivating herpes simplex virus infection was seen in 35% of the children who were seropositive prior to BMT. An extensive oral candidiasis was observed in 15% of the patients. All six children with a chronic GVHD exhibited changes in the oral mucosa 2-4 yr after transplantation such as erythma of the mucous membranes, tongue atrophy and also lichenoid changes in the buccal mucosa.