BACKGROUND Children undergoing bone marrow transplantation (BMT) are prone to develop severe gastrointestinal (GI) complications and metabolic imbalance which consequently impair their nutritional status. Nutritional support is an important adjunctive treatment during BMT. OBJECTIVE To assess GI complications, metabolic complications and nutritional outcome of children undergoing BMT with nutritional support intervention. METHODS Retrospective study of 20 children (median age 6.8 years, 11 males) undergoing BMT at Ramathibodi Hospital from March 1995 to July 2000 was conducted. Their medical records were reviewed. RESULTS The patients underwent autologous (n = 9) and allogenic BMT (n = 11). Median z-scores of weight for age, height for age and weight for height were 0.06 +/- 1.93, -0.55 +/- 1.18 and 0.48 +/- 1.94, respectively. Nineteen patients had vomiting for 9.8 +/- 5.5 days. Eighteen patients developed diarrhea for 9.6 +/- 7.2 days. The durations of vomiting and diarrhea, as a percentage of total hospital days, were 33.5 +/- 16.3 per cent and 30.4 +/- 17.0 per cent, respectively. There were no differences between the patients with autologous and allogenic BMT regarding these durations. All patients needed enteral and/or parenteral nutrition support for 21.0 +/- 7.7 days except for one patient who could take adequate oral intake. The duration of enteral nutrition support was not significantly different between the groups but the duration of parenteral nutrition support was significantly longer in the allogenic group. Metabolic complications were hypokalemia, hypophosphatemia and one case of arrhythmia secondary to hypomagnesemia. All patients developed febrile neutropenia but none developed catheter-related sepsis. The length of hospital stay was 30.5 +/- 10.2 days. The median z-score of weight for height on the day of discharge was 1.08 +/- 2.03. CONCLUSIONS Children undergoing BMT usually have GI symptoms of vomiting, diarrhea and mucositis as well as metabolic imbalances such as hypokalemia, hypophosphatemia and hypomagnesemia. Despite these complications, their nutritional status could be restored by proper nutritional support.