Thirteen patients with acute myeloid leukaemia (AML) who had been in unmaintained complete remission for at least 5 years following successful chemotherapy, were studied by marrow culture techniques regularly for periods of up to 5 years. In 10 patients, short-term clonal assay revealed the incidence of granulocyte-macrophage progenitor cells (GM-CFC) to be at the lower limit of the normal range; but three showed wide fluctuations in GM-CFC numbers. Moreover the generation of GM-CFC in long-term bone marrow culture (LTBMC) was impaired in eight of nine evaluable patients. These results suggest that, despite normal blood counts, impairment of haemopoiesis persists in patients who are long-term survivors of AML, and this does not improve with the passage of time.