In New York State, significant incidents involving the collection, processing, or transfusion of blood must be reported. Incident reports received over a 22-month period involving transfusion of blood to other than the intended recipient or release of blood of an incorrect group were analyzed. Among 1,784,600 transfusions of red cell components; there were 92 cases of erroneous transfusion that met study criteria (1/19,000). There were 54 ABO-incompatible transfusions (1/33,000); three of these (1/600,000) were fatal. Correction for underreporting of ABO-compatible errors resulted in an estimate of 1 per 12,000 as the true risk of transfusion error. National application of New York State data results in an estimate of 800 to 900 projected red cell-associated errors in the United States annually. The majority of reported errors occurred outside of the blood bank (43% resulted solely from failure to identify the patient and/or unit prior to transfusion and 11% resulted from phlebotomist error), while the blood bank was responsible for 25 percent of errors and contributed, with another hospital service, to 17 percent. The risk of transfusion of ABO-incompatible blood remains significant, and additional precautions to minimize the likelihood of such events should be considered.