The current FAB classification of acute myelogenous leukemia (AML) by morphologic, immunologic and cytogenetic features is described and the incidence of subtypes and variant forms of AML in study AML-BFM-87 is given. In general, lymphoblastic or myeloid blast populations can be identified by morphologic, cytochemical and immunologic parameters, while immunophenotyping facilitates diagnosis in biphenotypic leukemia. Furthermore, the AML subtypes M0 and M7 (defined in 1985 and 1991) can only be ascertained by the presence of lineage-associated markers. Whereas chromosome aberrations provide an insight into the neoplastic process and assure diagnosis. Thus, next to morphologic findings, immunologic and cytogenetic studies are essential for a definite diagnosis of AML.