With Prussian blue reaction nonhaemoglobin iron in the erythroblasts is demonstrable. Three pathological sideroblast types are recorded separately: abnormal intermediate type I and II sideroblasts and ring sideroblasts, representing increasing levels of sideroachrestic disturbance. This permits the classification of sideroachrestic disturbances into four degrees of seriousness. The frequency of a sideroachrestic disturbance in 47 untreated patients with acute myeloid leukaemia was 87%. Among 11 patients with preleukaemic condition, 8 had a disturbance of iron utilisation. In both preleukaemia and leukaemia mainly intermediate sideroblasts were present. All patients with preleukaemia developed leukaemia within 1-20 months. In the course of preleukaemic condition a slight increase of iron misutilisation was obvious when terminating in overt leukaemia. This could be of prognostic importance. After treatment, pathological sideroblasts disappeared only in 2 out of 15 patients with complete remission. There was no correlation between effect of therapy and course of iron misutilisation.