A patient with metastatic soft tissue sarcoma presented with a leukemoid reaction. The elevated white blood cell count was due to an increase in bands and mature segmented neutrophils. The degree of granulocytosis correlated with the tumor burden. There was no evidence of superimposed infection and the degree of bone marrow involvement by metastatic tumor was minimal. A cell line derived from the sarcoma produces granulocyte-macrophage colony-stimulating activity (CSA) in vitro. Although CSA could not be detected in serum, the findings in this patient suggest that the leukemoid reaction was due to tumor production of CSA.