A subdivision of ANLL into two categories occurring in the course of PV is proposed from the clinical, hematologic, and cytogenetic data: one resembling de novo ANLL with rapid initial evolution, easy classification into one group of the FAB nomenclature, and simple chromosome abnormalities; the other resembling induced leukemia, often with more progressive initial evolution, difficulty or impossibility of classification into one group of the FAB nomenclature, and complex chromosome abnormalities.