Myeloid colony growth from the peripheral blood of myelodysplastic (MDS) patients was assessed for abnormal in vitro response to haemopoietic growth factors (granulocyte colony-stimulating factor (G-CSF), macrophage colony-stimulating factor (M-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-1 (IL-1), interleukin-3 (IL-3)). Abnormal colony growth, increased or reduced, was observed with each of the factors. No specific growth pattern was related to any of the French-American-British classification (FAB) types of disease. MDS patients who had survived greater than 600 days after diagnosis (n = 34) showed significantly fewer abnormalities than patients assayed at the time of diagnosis (n = 37), the major difference being less frequent stimulation of colony growth. These findings indicate that the time of sampling relative to diagnosis needs to be considered when interpreting the in vitro response to growth factors of myeloid colonies from MDS patients.