Chronic myeloid leukemia consists of Philadelphia chromosome positive disease in 90% of cases, and a further 5%, although Philadelphia chromosome negative, exhibit bcr gene rearrangements consistent with the disease. The remaining 5% of cases have a heterogeneous clinical picture with a course unlike that of classical chronic myeloid leukemia, and may belong to different pathologic entities.

We report five cases belonging to the latter group, initially identified as Philadelphia chromosome negative, bcr non‐rearranged chronic myeloid leukemia, that developed progressive leucocytosis, absolute monocytosis, myelodysplasia, extramedullary hematopoiesis, and had evidence of myelofibrosis. These cases may represent a distinct clinical entity characterized by neutrophilic myelofibrosis, which can be identified prospectively by clinical and pathologic criteria. Standard therapy for treating chronic myeloid leukemia or idiopathic myelofibrosis may not be appropriate for this group.