Myeloid sarcoma is a rare extramedullary solid tumor consisting of immature myeloid cells and most commonly involving the bone, skin, lymph nodes, soft tissue, gastrointestinal tract and testis. Mediastinal myeloid sarcoma is very rare. There are two major types of myeloid sarcoma: granulocytic sarcoma and monoblastic sarcoma, according to immature cell type. Myeloid sarcoma is found in 2%-8% of patients with acute myeloid leukemia (AML). Myeloid sarcoma may develop before or concurrently with AML, or may be the initial manifestation of AML relapse in previously treated patients. Blast transformation of some form of myeloproliferative neoplasm or myelodysplastic syndrome may also manifest as myeloid sarcoma. A major differential diagnostic problem is isolated primary myeloid sarcoma without bone marrow and peripheral blood involvement, which may precede leukemic stage for months or years, and which is frequently misdiagnosed, mostly as malignant lymphoma. A case is presented of a 56-year-old female patient complaining of weakness, vertigo, dry cough and breathing difficulties. Clinical examination revealed enhanced vascular pattern on the right chest and right arm edema. Computed tomography (CT) of the thorax showed an expansive growth measuring 11 cm craniocaudally in the anterior mediastinum. Fine needle aspiration cytology of tumor mass yielded a scarcely cellular sample with individual atypical immature cells, fine chromatin structure and scarce cytoplasm with occasional granules and Auer rods. Considering the morphological, cytochemical and immunocytochemical characteristics of immature cells, the diagnosis of myeloid sarcoma was made and verified by histopathology of tumor biopsy sample. Immature cells were not found by analysis of bone marrow puncture sample, immunophenotyping of bone marrow cells and bone biopsy analysis. As immature cell proliferation was not detected in bone marrow and peripheral blood, while spread of the disease beyond the mediastinum was ruled out by imaging methods (CT, ultrasonography), it was decided to be a primary non-leukemic form of mediastinal myeloid sarcoma. Myeloid sarcoma should be taken in consideration on differential diagnosis of solid tumors because making an accurate diagnosis is necessary for timely initiation of appropriate therapy. Weakly expressed or lacking clear signs of myeloid differentiation may hamper morphological diagnosis. As isolated myeloid sarcoma is a very rare entity frequently resembling lymphoma in clinical presentation, it poses a major diagnostic challenge for both morphologists and clinicians.