A white blood cell count more than 50 x 10(9)/l, not related to bone marrow involvement, is termed leukemoid reaction. We report on the first case of an undifferentiated sarcoma of the lung associated with an intense paraneoplastic neutrophilic leukemoid reaction related to the production of granulocyte colony-stimulating factor (G-CSF). A radiography and a computed tomography scan of the chest revealed a well-limited voluminous and heterogeneous low-density mass of the left lung. The patient died of multiorgan failure related to uncontrolled progressive tumor growth after admission and two cycles of chemotherapy. The patient's G-CSF serum concentration was dramatically elevated (6,538 pg/ml) compared to serum levels observed in normal controls and patients with elevated leukocytosis (31 and 387 pg/ml, respectively). The G-CSF concentration dramatically increased after the first cycle of chemotherapy and during the subsequent neutropenia, as a result of the tumor lyses as well as of disruption of the physiological negative feedback mechanism. Adjunction of the patient's serum to CD34+ cell cultures induced a 12.3-fold increase in CD15+ cells, demonstrating the serum's capacity to induce myeloid differentiation.