In a paper by Ziegler et al (Ann Intern Med 116: 821-828, 1992), total parenteral nutrition supplemented with L-glutamine (TPN/GLN) was reported beneficial in patients receiving bone marrow transplantation (BMT) for hematologic malignancies. By using a similar protocol, we studied 29 patients with both hematologic malignancies and solid tumors, and with both allogeneic and autologous BMTs. In a double-blind, randomized approach, patients were given isocaloric, isonitrogenous TPN after BMT until they consumed 50% of their required diet orally. Total body water and extracellular water were measured before and after TPN in 10 patients. Total body water increased in patients receiving standard TPN and decreased significantly in patients receiving TPN/GLN. Length of hospital stay after BMT was significantly (5.8 days) less in patients receiving TPN/GLN. Incidence of positive bacterial cultures, clinical infections, and mortality did not differ significantly between the two groups. When the groups were subdivided into patients with hematologic malignancies and those with solid tumors, there were no significant differences in the above variables associated with TPN/GLN. In 17 of 30 additional hospitalized patients receiving standard TPN, substitution of TPN/GLN did not have discernible clinical or laboratory effects but appeared to be safe. Inclusion of patients with solid tumors and a higher mortality in our patients may have obscured beneficial effects of TPN/GLN observed by others.