Evidence shows that blood products, like other health care resources, are often used inappropriately, but the reasons for this have not been well studied. We conducted a face-to-face survey of 122 general surgeons, orthopedic surgeons, and anesthesiologists in three hospitals to evaluate the influence of several clinical and nonclinical factors on transfusion decision making. We found wide-spread deficiencies in physicians' knowledge of transfusion risks and indications. Each transfusion risk was estimated correctly by fewer than half of the physicians surveyed, and only 31% responded correctly to a set of four questions regarding transfusion indications. Attending physicians routinely had lower knowledge scores than did residents, yet they exhibited more confidence in their knowledge. Residents' transfusion decisions, however, were strongly influenced by the desires of their attending physicians, resulting in their ordering potentially inappropriate transfusions. Of the residents surveyed, 61% indicated that they ordered transfusions that they judged unnecessary at least once a month because a more senior physician suggested that they do so. These findings provide insights for the development of strategies to improve transfusion practices, which would address the dual concerns of quality of care and cost containment.