Data from animal studies suggests that glutamine supplementation may reduce the incidence or severity of cytotoxic-induced mucositis. This study examined the effect of glutamine supplements on mucositis in patients receiving bone marrow transplants. 24 patients were randomly assigned to receive an oral supplement of either glutamine or placebo (16 g/day) from the time of transplantation until discharge from hospital. There was no significant difference in the incidence of oral mucositis, assessed either by the patient (mucositis score: glutamine 17 vs. placebo 26.6) or an independent observer (glutamine 31.1 vs. placebo 32.3), or the number of days of diarrhoea (glutamine 3.1 vs. placebo 3.3 days). Similarly there were no significant differences in haematological indices (haemoglobin, WBC, platelets), the requirement for parenteral nutrition or hospital stay. Possible reasons for this negative result, which contrasts with those obtained in recent studies of patients receiving intravenous glutamine after bone marrow transplants, are discussed.