Loss of A, B, and H antigens from the surface of red blood cells has been a recurrent observation in patients with hematologic malignancy, particularly those malignancies in which the myeloid lineage is involved. To better understand this phenomenon, a 2-color flow cytometric method was developed to determine quantitative and qualitative alterations of A, B, and H antigens in patients with myeloid malignancies. Characteristic patterns, dependent on the genotype, were seen for healthy individuals from each of the blood groups. Fifty-five percent (16/29) of patients of blood group A, B, or AB had a proportion of red cells with decreased expression of A or B antigens compared with no changes in 127 healthy A, B, and AB individuals. In most cases, the changes were not detected by routine serologic typing. The loss of A or B antigens was the primary change in 28% (8/29) of patients. In 17% (5/29) of patients, loss of A or B antigens was an indirect consequence of loss of the precursor H antigen. Alterations involving both the H and the A or B antigens were seen in 10% (3/29) of patients. Loss of H was also detected in 21% (6/28) of group O patients whereas none of 51 healthy O individuals showed changes. Alterations of ABO antigens can now be considered a common event in myeloid malignancy. (Blood. 2001;97:3633-3639)