Chronic eosinophilic leukemia is a rare entity, characterized by eosinophilia of >1.5 x 10(9)/L, persisting for >6 months after exclusion of other reactive and neoplastic causes of eosinophilia, and occurring in association with a clonal cytogenetic abnormality. Various chromosomal abnormalities have been associated with chronic eosinophilic leukemia. Partial deletion of the long arm of chromosome 16 is a cytogenetic abnormality first reported 20 years ago in patients with acute myeloid leukemia associated with bone marrow eosinophilia (AML-M4Eo). We report a case of a 45-year-old African-American male with hepatitis C and sustained peripheral blood eosinophilia who presented with gross hematuria, dyspnea on exertion, chills, decreased appetite and weight loss of 40 pounds associated with hepatosplenomegaly and lymphadenopathy. Bone marrow biopsy showed clonal cytogenetic abnormality consisting of deletion of the long arm of chromosome 16 (16q22). Philadelphia chromosome t (9;22) and polymerase chain reaction (PCR) analysis for C-kit and platelet-derived growth factor receptor-alpha (PDGFRA) mutations were negative. The patient was treated with imatinib at 400 mg/d with improvement of symptoms, reduction of lymphadenopathy and normalization of the eosinophil count. To our knowledge, this is the first case report of isolated del (16) (q22), a cytogenetic abnormality associated with AML-M4Eo, occurring in chronic eosinophilic leukemia. Whether this cytogenetic abnormality might represent a prodromal phase of AML-M4Eo is not known. In addition, the role of hepatitis C in inducing clonal proliferation of eosinophils is unclear.