Hematopoietic progenitor cells are capable of forming colonies of mature blood cells in semisolid media in response to specific growth factors. Colony assays have been extensively used for many years to study normal and malignant hematopoiesis in vitro. In fact, these assays have provided an excellent research tool for investigating growth and differentiation of progenitor cells in response to positive and negative regulators of hematopoiesis. However, apart from their role in basic research, colony assays are also widely used in routine clinical practice in the diagnosis of various hematologic disorders, such as aplastic anemia, myelodysplastic syndromes and myeloproliferative disorders. This review summarizes our current knowledge on the diagnostic value and prognostic significance of the growth of progenitor cells in peripheral blood and bone marrow in patients with myeloid malignancies.