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The presence of leukocytosis associated with non-hematological malignancy after ruling out other causes is defined as paraneoplastic leukemoid reaction (PLR). PLR is a rare manifestation of various solid tumors. It is associated with poor prognosis unless receiving effective antineoplastic treatments. Case Report: A 72-year-old female was referred to a hematologist/oncologist for the evaluation of leukocytosis with neutrophilia. Initial workup was unremarkable; however, she had progressively worsening leukocytosis with neutrophilia, associated with severe anemia and dysphagia. Computed tomography (CT) scan revealed wall thickening at the gastroesophageal junction (GEJ) and multiple hypodensities of the liver. Esophagogastroduodenoscopy (EGD) confirmed the diagnosis of GEJ tumor and biopsy returned as adenocarcinoma with human epidermal growth factor receptor 2 (HER2) overexpression. Leukocytosis resolved after the first round of chemotherapy and the patient remains progression-free with the addition of trastuzumab to her chemotherapy regimen. Conclusions: We report a rare case of PLR caused by GEJ adenocarcinoma. This is the first case of PLR in a patient with metastatic GEJ adenocarcinoma with HER2 overexpression in the Caucasian population. It is important to workup leukocytosis promptly, to keep malignancy in the differential diagnosis and to seek early hematology/oncology consultation.

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