We describe a case of idiopathic myelofibrosis with total neutrophil myeloperoxidase deficiency. The combination of this enzymatic defect with myelofibrotic changes in the nuclear shape of neutrophils confers a peculiar appearance on leukograms produced by a Technicon H*1. The clinical course of the disease was shortened by recurrent infections that may be ascribed, at least in part, to reduced leukocyte microbicidal ability.