Granulocyte colony-stimulating factor (G-CSF) is now widely used in patients with malignant disorders receiving intensive chemotherapy to increase leukocyte count and to upregulate phagocyte function during neutropenia. Monocytosis associated with G-CSF has been reported in anecdotal literature. We report two cases of pseudoleukemia secondary to G-CSF administration. Both patients initially presented with myelodysplastic syndrome with chromosome 7 abnormalities that evolved into acute myeloid leukemia. Case one had deletion 7q while case two initially had monosomy 7 and subsequently developed a balanced translocation between the short (p) arm of chromosome 1 and long (q) arm of chromosome 15. Following the induction chemotherapy and G-CSF administration, both of these patients developed pseudoleukemia. Patient 1 had white blood cell (WBC) count of 26 x 10(9)/l with 72% monocytes, while patient two had WBC of 14.1 x 10(9)/l with 30% monocytes. In both patients the monocytosis resolved after the discontinuation of G-CSF therapy. In summary, patients treated with G-CSF should be followed closely. In those cases with pseudoleukemia discontinuation of the drug with no supplemental chemotherapy is probably enough to control the atypical monocytosis.