We used the human placental conditioned medium stimulated single layer agar culture technique to study the in vitro growth of marrow cells from 62 adult patients with acute myeloid leukemia (AML). Bone marrow cells were cultured from 50 patients at the time of initial diagnosis, 19 patients in early remission and 20 patients during their full complete remission. Marrow cultures from untreated patients exhibited heterogeneous growth patterns ranging from complete growth failure to excessive microcluster formation. We classified the growth patterns into 4 groups: (1) Gr I: normal growth, (2) Gr II: no growth, (3) Gr III: decreased growth, (4) Gr IV: excessive growth of microclusters. At presentation, none had Gr I growth; Gr II growth was observed in 23; Gr III in 14 and Gr IV in 13. A predominance of no growth were seen in M1 and M3 subtypes, while Gr IV growth was more commonly observed in M2 or M4 subtype. We were unable to correlate the culture findings with age or white cell count. The present results not only indicated that AML at diagnosis was characterized by abnormal granulopoiesis but also demonstrated that leukemic progenitor cells were heterogeneous with different capacities to express their proliferating potential in vitro. Except few with decreased growth, the growth characteristics generally returned to normal with successful remission induction. Both Gr II and Gr IV growth patterns were not observed either in early remission or during full complete remission.