Oral mucositis is a frequent side effect of myeloablative chemo- and radiotherapy preceding stem cell transplantation. It causes pain, poor food intake, and is a port of entry for infection. We studied whether GM-CSF applied topically in the oral cavity can prevent or ameliorate this mucositis. In 36 consecutive patients undergoing a stem cell transplantation, we performed a double-blind placebo-controlled study of 300 micrograms GM-CSF in a 2% methylcellulose gel daily versus a 2% methylcellulose gel alone. Both were locally applied in the oral cavity. The primary end-point was mucositis as measured by the WHO toxicity scale for mucositis, oral assessment scale, and a subjective pain scale, all scored daily. The secondary end-points were need to give parenteral nutrition and morphine, incidence of fever and infections, and duration of neutropenia and hospitalization. No differences were found in the median subjective pain scores, WHO scores, and oral assessment scores between the placebo and the GM-CSF groups. In both groups, nine patients required morphine for pain control. Ten patients in the placebo group and 11 in the GM-CSF group received parenteral nutrition. Documented infections, use of broad-spectrum antibiotics, and number of days with fever were similar in the placebo and the GM-CSF groups. The duration of neutropenia below 0.5 x 10(9)/l (median 14.5 days in the placebo group versus 17 days in the GM-CSF group) and the duration of hospitalization (28.5 versus 29 days) was also not significantly different. We found no beneficial effect of 300 micrograms GM-CSF dissolved in a 2% methylcellulose gel applied locally for chemo- and radiotherapy-induced mucositis in patients undergoing a stem cell transplantation.