Two patients with acute nonlymphocytic leukemia (ANLL) (FAB-M4) and t(6;9)(p23;q34) are described. Immunological marker analysis revealed a phenotype of HLA-DR+/partly terminal deoxynucleotidyl transferase (TdT)+/CD13+ in both cases and CD33 positivity in one. The expression of CD13 and CD33 by TdT-positive cells was demonstrated by double immunofluorescence staining. Although it has been postulated that TdT plays a role in gene rearrangement, Southern blot analysis performed in one leukemia revealed that both the T cell receptor beta chain genes and the immunoglobulin heavy chain genes were in germ line configuration. Since we could not detect CD13+/TdT+ cells and CD33+/TdT+ cells in control bone marrow samples, double marker analysis was used to detect low numbers of residual leukemic cells during follow-up of one patient. A gradually increasing percentage of CD33+/TdT+ cells was detected in the bone marrow in a period of 6 months before hematological relapse. Although the t(6;9) may not be correlated to a specific French-American-British subtype, it may be associated with TdT-positive ANLL. Since TdT-positive ANLL seems to have a poor outcome, detection of TdT expression in ANLL patients is particularly important for diagnostic purposes. In addition, our results indicate that double immunological marker analysis for a myeloid marker and TdT allows detection of residual disease during follow-up.