Human bone marrow contains cells which form leukocyte colonies in semisolid culture media. Each leukocyte colony arises from a single colony-forming cell which is thought to be a unipotential stem cell, and which is subject to regulation in vitro by colony-stimulating factor. In acute myelogenous leukemia variable abnormalities in colony formation by marrow cells occur. Usually colony formation either fails to occur or the colonies that are formed are small and contain fewer than 50 cells. Similar abnormalities have been described in bone marrow dysfunction preceding overt leukemia. Usually remission of leukemia is accompanied by improved cloning by marrow cells. In this study three patients are reported in whom remission was associated with impaired cloning, and one of these patients has remained in continuous remission for a further 18 months. These observations suggest that remission status is not necessarily associated with repopulation of the bone marrow by normal hematopoietic cells.