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A case of sarcomatoid collecting duct carcinoma (CDC) of kidney is presented, in which the diagnosis was made cytologically with voided urine and renal pelvis lavage. Cytology of hemorrhagic voided urine revealed highly atypical adenocarcinoma cells with reminiscent ductal structure, which suggested CDC as the most likely diagnosis. Computed tomography and magnetic resonance imaging demonstrated a left renal tumor, and selective lavage of left renal pelvis yielded spindle-shaped, highly atypical cells that indicated sarcomatoid carcinoma. The diagnosis of renal cancer with urine cytology is challenging because of small number of tumor cells in the urine, which are often associated with degeneration. As the urinary cytologic findings of sarcomatoid CDC have not been reported, the characteristic cytologic findings of sarcomatoid CDC are described in detail, and the differential diagnoses with diagnostic pitfalls were discussed. Diagn.