Eosinophilia associated with solid tumors is an infrequent occurrence. The pathogenesis of tumor-associated eosinophilia is not well understood. Interleukin-5 (IL-5) is a cytokine that has been implicated in the development of eosinophilia in mice and humans. However, there is little data associating IL-5 production with eosinophilia in the presence of tumor. We report, in a patient with locally advanced NSCLC, the presence of excessive eosinophilia and elevated serum IL-5 levels at diagnosis. Immunohistochemical staining of the primary tumor showed large amounts of intracellular IL-5. Both eosinophil count and IL-5 levels normalized after surgical removal of the tumor. Tumor-associated eosinophilia observed in this case is mediated by IL-5. The production of IL-5 is related to the presence of the tumor.