Fifteen patients received irradiation after combined chemotherapy with high-dose busulfan followed by autologous or allogeneic bone marrow transplantation. Of nine patients irradiated between day 30 and day 70 after their engraftment, seven developed an increased radiation response in the skin: four showed enhanced reactions during irradiation, and three had a total or severe definitive alopecia more than 16 months after CNS irradiation. Six patients were irradiated after day 70; they had either normal reactions or none at all within the cutaneous radiation portals. The possibility that busulfan might be a radiosensitizer is raised.