The in vitro granulocyte colony-forming ability of bone marrow from 19 patients with acute myelocytic leukemia (AML) was studied at the time of initial diagnosis and serially. Initially 18 of the 19 patients grew scant numbers of colonies, one produced large numbers of colonies. When clinical remission was attained and leukemic blasts in the marrow were reduced to <5%, colony forming ability usually returned to normal. Eight such patients who achieved clinical remission were serially studied during remission. Normal in vitro colony formation ensued in five of the eight patients studied. Three patients did not produce normal numbers of colonies, and it was noted that these patients relapsed within 2 mo. The repeated observation of normal numbers of granulocyte colonies appears to distinguish patients whose remissions are stable from those patients who relapse quickly.