Twenty-three cases of delayed hemolytic transfusion reaction (DHTR) occurring at the Mayo Clinic from 1964 through 1973 are reviewed. Nineteen patients had clinical manifestations of hemolysis, of which fever was the most frequent presenting symptom. The degree of hemolysis served as an index of morbidity. In four cases, there was oliguria, two of these patients experiencing renal shutdown. In one case, hemolysis led to a disseminated intravascular coagulation syndrome. Death occurred subsequent of DHTR in three patients. The direct antiglobulin test was positive in all but one case; this finding coincided with elevated unconjugated bilirubin in 14 cases and decreased haptoglobin levels in 15 cases. Anti-Jka antibody accounted for somewhat more than one-third of reactions and, along with anti-E, c, D, Fya, and K antibodies accounted for 91 per cent of cases.