Myelodysplasia (MDS) and acute myeloid leukemia (AML) are heterogeneous, closely associated diseases arising de novo or following chemotherapy with alkylating agents, topoisomerase II inhibitors, or after radiotherapy. Whereas de novo MDS and AML are almost always subclassified according to cytogenetic characteristics, therapy-related MDS (t-MDS) and therapy-related AML (t-AML) are often considered as separate entities and are not subdivided. Alternative genetic pathways were previously proposed in t-MDS and t-AML based on cytogenetic characteristics. An increasing number of gene mutations are now observed to cluster differently in these pathways with an identical pattern in de novo and in t-MDS and t-AML. An association is observed between activating mutations of genes in the tyrosine kinase RAS-BRAF signal-transduction pathway (Class I mutations) and inactivating mutations of genes encoding hematopoietic transcription factors (Class II mutations). Point mutations of AML1 and RAS seem to cooperate and predispose to progression from t-MDS to t-AML. Recently, critical genetic effects underlying 5q-/-5 and 7q-/-7 have been proposed. Their association and cooperation with point mutations of p53 and AML1, respectively, extend the scenario of cooperating genetic abnormalities in MDS and AML. As de novo and t-MDS and t-AML are biologically identical diseases, they ought to be subclassified and treated similarly.