A 65-year-old male from Nigeria, admitted with dehydration and painful oral ulcers, is seen by allergy/immunology for hypereosinophilia.The patient had idiopathic eosinophilia (mean 5,000 cells/uL) ongoing for nine years. There were no evident drug or infectious causes. The eosinophilia peaked at 28,000 cells/µL, accompanied by parotid swelling, pustular skin rash, and elevated serum-IgG4 level (400 mg/dL). Rheumatology evaluated for IgG4-related disease and hypereosinophilic syndrome. This work-up demonstrated elevated IgE (39,000 IU/mL) and tryptase (40 ng/mL) levels. Bone marrow biopsy showed a hypercellular marrow with erythroid hyperplasia and eosinophilia. Submandibular gland, skin, and excisional lymph node biopsies demonstrated IgGsecreting plasma cells, lymphocytic/eosinophilic infiltrates, and lymphoid hyperplasia, respectively. All biopsies stained negative for IgG4. Treatment for idiopathic hypereosinophilia was initiated with corticosteroids, rituximab, and mycophenolate mofetil (MMF). Symptomatic improvement was attained on MMF (1000 mg twice daily) with the addition of mepolizumab (300 mg every 4 weeks).He unfortunately progressed to high fevers with elevated ferritin (7,286 ug/L) and soluble IL-2Ra (31,276.7 pg/mL) levels. He did not meet full criteria for hemophagocytic lymphohistiocytosis and improved after corticosteroids. EBV PCR from blood was >220,000 IU/mL.Dedicated immune work-up showed a non-specific population of lymphocyte-like cells without clear lineage markers (CD2dim/- surface CD3-CD4-CD8-CD7-CD56-CD49d-CD294dim) on peripheral blood flow cytometry, with otherwise normal T, B, and NK cell enumeration. There was reduced cytotoxic killing function of CD8+ T cells and NK cells by chromium-51 release assay. Sorted peripheral blood mononuclear cells demonstrated positive EBV viral load in B cells (1,018,239 IU/mL), T cells (857,418 IU/mL),NK cells (237,459), and myeloid cells (532,641 IU/mL). EBV-encoded RNA was positive within lymph node T cells, most consistent with a diagnosis of T-cell CA-EBV. The Invitae 407 gene immunodeficiency panel was sent; however, a pathogenic variant was not identified. The patient was seen by hematology/oncology with plans for hematopoietic stem cell transplant. Unfortunately, he succumbed to complications from CA-EBV and multiorgan failure.We describe a male with lymphocytic-variant hypereosinophilic syndrome and a longstanding constellation of inflammatory and lymphoproliferative symptoms due to CA-EBV. His course initially eluded his care team and only after careful multidisciplinary discussion was a diagnosis reached.