Objective: The purpose of this study was to determine the most appropriate dose of oral glutamine to use in a further clinical study in paediatric oncology patients. Design: This was a phase I, pharmokinetic study. Setting: The study was carried out at The Yorkshire Regional Centre for Paediatric Oncology and Haematology, St James's University Hospital, Leeds, UK. Subjects: Thirteen patients undergoing treatment for paediatric malignancy participated in this study. All 13 completed the study. Interventions: The most appropriate dose was determined by patient acceptability and by plasma glutamine and ammonia levels measured at timed intervals after ingestion of a single glutamine dose. Results: Doses of 0.35, 0.5 and 0.65 g=kg were well tolerated with no untoward plasma glutamine and ammonia levels. One patient was recruited to a higher dose of 0.75 g=kg, but the plasma glutamine and ammonia levels peaked at 2601 and 155 mmol=l, respectively. The ammonia level was greater than the acceptable upper limit. It was difficult to disperse the glutamine adequately at this dose, resulting in the suspension being found to be unpalatable and therefore no further patients were recruited at this dose. Conclusion: It was concluded that 0.65 g=kg is a safe dose of glutamine to use in a clinical study in paediatric oncology patients. Sponsorship: Scientific Hospital Supplies UK Ltd provided the L-glutamine and financial help for the biochemical analysis.