Gradual increase of CD38 on cells expressing CD34 characterizes the early cell differentiation pathway of normal human hematopoietic progenitors. In this study the coordinated expression pattern of CD34 and CD38 was assessed on leukemic blasts from bone marrow aspirates of 95 patients with newly diagnosed acute myeloid leukemia (AML). Expression was divided into six categories analogous to the differentiation pathway of normal bone marrow. The CD38 antigen was expressed on the leukemic cells of all patients and CD34+ leukemic cells were found in 79 patients (83%). In 93 patients, the leukemic cells were found along the differentiation pathway defined by CD34 and CD38. In 33 of the 93 patients, a part of the CD34+ cells did not express the CD38 antigen (categories 1 and 2). In another 33 patients, all CD34+ cells expressed CD38 (categories 3 and 4). In the remaining 27 patients, only cells were found which dimly expressed CD34 or did not express CD34 (categories 5 and 6). Of the 93 patients, 88 were treated with intensive chemotherapy according to the protocol of the German AML Cooperative Group. Of these, 21 died early and were not evaluable for treatment response. Complete remission was achieved in 14 of 22 patients (64%) in categories 1 and 2, in 19 of 26 patients (73%) in categories 3 and 4, and in 18 of 19 patients (95%) in categories 5 and 6. The event-free survival was significantly longer in patients of categories 5 and 6 compared to patients in categories 1 and 2 (p less than 0.01) and categories 3 and 4 (p less than 0.05), respectively. We conclude that in the majority of AML patients the immunophenotype of leukemic cells follows the early cell differentiation pathways defined by coordinated expression of CD34 and CD38 similar to that of normal hematopoietic progenitors. The presence of cells in the late cell differentiation stages (CD34+/-, CD38 /+) identifies patients with a higher complete remission rate and longer complete remission duration.