Apparent hematological symptoms rarely dominate the clinical picture of an underlying non-hematological malignancy. Malignancy-associated eosinophilia can result from clonal or non-clonal proliferation of eosinophils. Here, we report the case of a 59-year-old man with metastatic adenocarcinoma of the lung with an unknown primary tumor site, which presented as hypereosinophilia, anemia, lymphadenopathy, weight loss, and malaise. Bone marrow biopsy disclosed metastatic adenocarcinoma positive in immunohistochemistry for cytokeratin 7. Further assessment of specimens obtained from the bronchoalveolar lavage and biopsy of the mediastinal lymph nodes confirmed the diagnosis of the metastatic lung cancer, although the primary tumor site remained undiscovered. This case underlines that eosinophilia may represent a rare primary manifestation of an undetected malignancy, and it is thus important to consider this as part of the differential diagnosis in patients presenting with unexplained eosinophilia.