Babesia microti, the primary cause of human babesiosis in the United States, is an intraerythrocytic parasite endemic to the Northeast and upper Midwest. Published studies indicate that B. microti increasingly poses a blood safety risk. The American Red Cross Hemovigilance Program herein describes the donor and recipient characteristics of suspected transfusion-transmitted B. microti cases reported between 2005 and 2007.Suspected transfusion-transmitted Babesia infections were reported by transfusion services or were discovered through recipient-tracing investigations of prior donations from donors with a positive test for B. microti in a serologic study. Follow-up samples from involved donors were tested by Babesia-specific immunofluorescence assay, Western blot, and/or real-time polymerase chain reaction analysis.Eighteen definite or probable B. microti infections, including five fatalities, were identified in transfusion recipients, 16 from hospital-reported cases and two through serologic lookback studies. Thirteen recipients were 61 to 84 years old and two were 2 years old or younger. Two recipients had sickle cell disease and four were known to be asplenic, including one with sickle cell disease. Seventeen antibody-positive donors were implicated; 11 (65%) were residents in Babesia-endemic areas, while four (24%) nonresident donors had a history of travel to endemic areas.Transfusion-transmitted B. microti can be a significant cause of transfusion-related morbidity and mortality, especially in infant, elderly, and asplenic blood recipients. These data demonstrate the need for interventions, in both endemic and nonendemic areas of the United States, to reduce patient risk.