We present the differential diagnosis for a Coombs-positive immune hemolysis having onset during hospitalization and, in particular, during the postoperative period. The stimulus for this article was a delayed hemolytic transfusion reaction (DHTR) due to anti-U following open-heart surgery. The initial clinical and serologic findings led us to consider other causes of immune hemolysis which are reviewed in this article. To our knowledge, this is the fourth case of a DHTR due to anti-U to be reported in the medical literature. ImmunohematoIogv 1993;9:41-46.