From March 1997 through November 1997, 8 allogenic bone marrow transplant (BMT) patients developed Stenotrophomonas maltophilia bacteremia on the hematology service at UCLA Medical Center (Los Angeles). Five of these patients had undergone transplantation during the same hospitalization that S. maltophilia bacteremia was detected (case patients). Compared with 7 concurrently hospitalized allogenic BMT patients (control patients), the 5 case patients were more likely to have been hospitalized in room A (P=.045), to have severe neutropenia on the culture date (P=.028), to have a longer duration of severe neutropenia (P=.05), to have severe mucositis (P=. 028), and to have received total parenteral nutrition (P=.028). Pulsed-field gel electrophoresis revealed that 2 of 3 isolates from case patients hospitalized in room A were identical. In allogenic BMT patients, severe neutropenia and severe mucositis may promote infection with S. maltophilia by impairing host defenses.