In a retrospective study of 105 patients with non-small cell lung cancer during a 5-year period, 43 had leukocytosis. In 19 of the 43 patients, no clear cut etiology for the leukocytosis was apparent and it was attributed to the tumor itself. In these 19 patients, absolute neutrophilia was detected in 13, eosinophilia was present in three, and eleven exhibited concomitant thrombocytosis. Tumor-associated leukocytosis occurred predominantly, and eosinophilia exclusively, in patients with large cell pulmonary neoplasms. These results suggest an unusual myeloproliferative stimulus in this type of cancer. It may result from tumor cell production of hemopoietic growth factors such as granulocyte-macrophage colony-stimulating activity; however, additional studies are needed to elucidate the underlying mechanism(s), and to determine whether this is a peculiar characteristic of the cells that comprise large cell undifferentiated carcinoma of the lung.