Myeloid sarcoma (MS) is an extramedullary tumor of immature myeloid cells. We analyzed 131 patients with MS, including: (1) de novo MS; (2) MS with concomitant acute myeloid leukemia (AML); (3) MS following myelodysplastic syndrome, myeloproliferative neoplasm, or chronic myelogenous leukemia; and (4) MS as a recurrence of AML. The most common development site was the lymph node. Testicular lesions were statistically more frequent in MS as a recurrence of AML than in other types of MS (P=0.0183). MS tended to lack myeloid markers (myeloperoxidase was present in 63.2%, CD68 in 51.3%, CD13 in 48.7%, and CD33 in 48.7% of patients) and express T-cell markers such as CD3 (20.7%) and CD5 (34.2%). All T-cell marker-positive MS cases were negative for the αβ and γδ T-cell receptors on immunohistochemistry. Underlying myelodysplastic syndrome or myeloproliferative neoplasm was a poor prognostic factor (vs. de novo MS: P=0.0383; vs. MS with concomitant AML: P=0.0143). However, there was no statistical difference in prognosis between de novo MS and MS with concomitant AML (P=0.288). There were no significant differences in prognosis between the prognoses of T-cell marker-positive and T-cell marker-negative MS cases. In addition, CXCR4 expression was a poor prognostic factor in MS (P=0.0229). This study involves the largest MS cohort to date and expands the clinical and pathologic knowledge of the disease.