Clinicians have usually considered malignancies during follow up of patients who have infectious diseases as a pre-diagnosis. However, malignancy and an infectious disease are seen together more rarely, with the exception of immunosuppressed patients. This presentation is a case report followed up for fever of unknown origin. The patient was admitted to the hospital with the symptoms of fever, weight loss, abdominal pain and weakness. Anemia and hypergamaglobulinemia by biochemical analyses and splenomegaly by total body computed tomography were detected. Amastigotes were seen in bone marrow aspiration smears and promastigotes were isolated in NNN medium. At the end of the Liposomal Amphotericin B treatment, control bone marrow aspiration was applied. Leishmania amastigotes were not seen, while patient was diagnosed as diffuse B cell lymphoma pathologically.