In a randomized trial we studied the impact of providing total parenteral nutrition (TPN) to bone marrow transplant (BMT) patients during their cytoreductive therapy, and for 4 weeks following BMT, on 8 parameters of outcome. A total of 137 patients over 1 year of age and with normal nutritional status were randomized either to receive TPN starting one week prior to transplant or to receive hydration with a 5% dextrose solution containing electrolytes, minerals, trace elements, and vitamins. TPN was ultimately required by 40 of the 66 control patients when nutritional depletion was documented. Average total calorie and protein intake was significantly higher for the TPN group than for the control group. Minimum follow-up was 1 year and median was 2 years. Overall survival, time to relapse, and disease-free survival were significantly improved in the TPN group. Engraftment, duration of hospitalization, and incidences of acute and chronic graft-vs.-host disease and bacteremia were not different. Thus TPN during BMT had a positive effect on long-term outcome. Prophylactic nutritional therapy appears to be indicated even for well-nourished individuals during cytoreduc-tion and BMT.