Blood and bone marrow samples from 106 patients with Acute Myeloid Leukemia were reviewed independently by three observers according to the FAB proposals. Without a cytochemical stain, complete agreement occurred in 75% of cases. The major discrepancies were between M1 and M5 poorly differentiated (4 cases) and M2 and M4 (12 cases). These discrepancies disappeared with cytochemistry. Others non-concurrences were between M1 and M2 (4 cases) M2 and M6 (3 cases), M4 and M5 differentiated (2 cases), M2 and RAEB (1 case). To solve these latter discrepancies some points of the FAB classification require clarification.