Smears of peripheral blood and bone marrow from 17 patients with acute myeloid leukaemia in relapse were compared with smears made at presentation. We found a change of more than 10 in Maturation Index (MI) in the bone marrow smears of 14 out of 17 patients (82%). Decreases and increases were equally represented. In 5 cases the changes caused assignment to another FAB-group.