Two cases of polycythemia vera (PV) had transition to a hematological condition compatible with chronic neutrophilic leukemia (CNL) 17 and 8 years after diagnosis, respectively. One patient was treated with carboquone followed by hydroxyurea (HU) and the other with HU during PV phase. On transition, both had neutrophilia with white blood cell count above 40,000/microl, elevated neutrophil alkaline phosphatase activity, splenomegaly, normal karyotype without bcr-abl rearrangement. Busulfan was temporally effective in controlling the neutrophil count. However, one patient progressed to the so-called spent phase and the other subsequently had multiple transitions between PV and CNL. These cases may represent a form of uncommon evolution of PV and support the contention that CNL is a type of myeloproliferative disorder and that at least some CNL cases have derangement at the hematopoietic stem cell level.