Bone marrow and/or blood cells from 81 patients with acute leukaemia, including 50 patients with acute myeloid leukaemia, were cultured in vitro using the agar culture method. The cells were cultured either in a single layer assay or with a normal source of colony stimulating factor (CSF) included in the culture. In almost all cases an abnormal growth pattern was seen, ranging between no growth and an excessive number of clusters. Immature granulocytic cells and macrophages were the dominating cell types in clusters and colonies from 14 out of 16 patients with acute myeloid leukaemia. In the patients with acute myeloid leukaemia the remission induction rate was found to be related to the cluster incidence. Thus, 75% of the patients with a cluster incidence between 0-100 obtained a complete remission (CR), while only 31% of the patients with a high cluster incidence (greater than 1000/2 x 10(5) cells) obtained a remission. Cluster formation in the unstimulated cultures of CSF-sensitive cells did not seem to be of prognostic value. The correlation between prognosis and in vitro growth in patients with acute myeloid leukaemia stresses the need of obtaining more insight in those factors other than drug sensitivity, which are of importance for the therapeutic response.