The molecules detected by CD34 and CD56 monoclonal antibodies are simultaneously expressed in approximately 20% of childhood acute myeloid leukemia (AML) cases, and this phenotype is associated with t(8;21)(q22;q22) karyotype. By contrast, bone marrow samples from normal donors (n = 5) and patients with CD56- malignancies in remission (n = 8) contained fewer than 1 in 10,000 CD34+/CD56+ cells. CD34+/CD56+ cells were readily identified when leukemic blasts were admixed with normal bone marrow cells at a 1:10(4) ratio. Cells expressing both markers (0.01-0.8% of mononuclear cells) were also found in bone marrow samples from two of three children with CD34+/CD56+ AML studied, who were in remission by morphologic criteria. In one of these patients, detection of residual disease by flow cytometry anticipated overt hematologic relapse. A second patient, in whom minimal residual disease was detected prior to and following autografting, died of unrelated causes while in morphologic remission. The third patient had no detectable residual disease prior to and following autografting, and is still in morphologic and immunologic remission 100+ days post-transplant. The expression of CD56 on CD34+ cells is leukemia-associated and offers a means of identifying extremely small numbers of these cells by flow cytometry. This sensitive approach can now be used to assess the efficacy of treatment and detect early relapse in patients with CD34+/CD56+ AML.