Renal medullary carcinoma is an aggressive neoplasm of the renal pelvis arising in patients with a history of sickle-cell trait. The authors report a case of renal medullary carcinoma with positive urinary cytology. Due to the location of the tumor in the renal pelvis and the loosely cohesive nature of poorly differentiated neoplasms, the presence of renal medullary carcinoma in a urinary cytology specimen is not surprising. The cytologic characteristics as well as the ultrastructural features are described. Diagn. Cytopathol. 1998;18:276–279. Published 1998 Wiley-Liss, Inc.