Expression of the normally cryptic blood group antigen Tn has occasionally been reported in hematologic disease, but the true frequency of this change is not known. A mouse monoclonal antibody (FBT3) and immunohistochemistry were used to examine expression of the Tn antigen. Expression was not detected in 35 normal bone marrow aspirates examined, but it was detected in 5 of 725 abnormal bone marrow aspirates, including 2 (3.6%) of 55 cases of de novo acute nonlymphocytic leukemia and 2 cases that terminated in acute nonlymphocytic leukemia. In two patients, one with acute myeloblastic leukemia and the other in blast transformation of chronic myeloid leukemia, the Tn antigen was expressed on 2 percent of blast cells. In one case of non-Hodgkin's lymphoma, 4 percent of normal myeloid cells expressed the antigen. In the other two cases, one of acute myelomonocytic leukemia and the other of myelodysplasia, only 2 to 8 percent of myeloid and erythroid cells initially were Tn positive. Subsequent serial immunohistochemical studies of bone marrow aspirates and peripheral blood in these two cases showed increasing numbers of Tn-positive erythroid and myeloid cells 8 to 12 months before polyagglutination was detected serologically. Tn-positive cells increased to > 90 percent in the terminal phase in both cases of both diseases. The results suggest that Tn expression in these two patients may have conferred a growth advantage to the cells and could be related to disease progression.