The cytologic evaluation of a case of acute leukemia proceeds in two stages: 1) assigning the leukemia to one of two major classes, acute lymphocytic leukemia (ALL) or acute nonlymphocytic leukemia (ANLL); and 2) determining the proper subclassification of ALL or ANLL to which the case belongs. Although features such as nuclear morphology, number and nature of nucleoli, and nuclear/cytoplasmic ratio are useful criteria, the major morphologic features which distinguish ANLL cells from ALL cells are the presence of azurophilic granules and/or inclusions derived from fusion of these granules (Auer bodies). In most instances these can be visualized in the conventional blood film or bone marrow preparation. Sometimes, however, granules and Auer bodies are too small or too few to be seen with the light microscope. In such cases histochemical or ultrastructural studies will aid in proper classification.