Hematopoietic cell transplantation is an elective procedure that results in prolonged immune suppression and high treatment-related morbidity and mortality. Transplant centers and physicians use a variety of prophylaxis and monitoring strategies to prevent or minimize complications. Little is known about the variability in these practices. We conducted an international Internet-based survey of 526 physicians to describe the spectrum of supportive care practices employed. Consistency in pretransplant cardiac (96%) and pulmonary (95%) screening, informed consent documentation (93%), and use of antifungal prophylaxis (92%) was observed. Greater heterogeneity was seen in use of myelogenous growth factors, empiric antibiotic therapy, protective isolation procedures, posttransplant monitoring, and environmental and social restrictions. Although some practice differences were associated with physician characteristics and transplant type, most practice variation remained unexplained. These results suggest a need for well-designed observational and interventional studies to provide data about which supportive care practices improve outcomes. For practices proved to be beneficial, publication of guidelines and incorporation of monitoring into quality improvement initiatives may help standardize practices.