Myeloid sarcomas (MS) are rare extramedullary hematological tumors which generally occur during the natural course of acute myeloid leukemia or chronic myeloid leukemia. Rarely, their onset precedes peripheral blood and bone marrow manifestations of disease. Common sites of involvement are skin, bone, soft tissue, lymph nodes, reproductive or digestive organs, and central nervous system.

Herein, we report the case of a 72-year-old man affected by JAK2 V617F mutated myeloproliferative neoplasm who developed MS involving collecting system of both kidneys. MS and MS-related obstructive nephropathy were the first signs of the acute evolution of a known chronic hematological malignancy, preceding by some weeks the onset of leukocytosis.