We studied the efficacy of mesna as a protectant for urotoxicity in pediatric patients receiving chemotherapy including oxazaphosphorines. Nineteen patients with malignant diseases (5 neuroblastoma, 3 acute lymphocytic leukemia, 4 acute non-lymphocytic leukemia, 2 non-Hodgkin lymphoma, 3 osteosarcoma and 2 rhabdomyosarcoma) were treated with a total of 106 courses of therapy between June of 1986 and May of 1989. Of these, no gross hematuria were seen. Microhematuria transiently occurred only in 2 courses (5%) of 1 patient (2%). These data indicated that mesna was highly effective for urotoxicity of oxazaphosphorines without any side effects, especially in pediatric patients.