BACKGROUND: Although urine cytology is utilized in the detection of urothelial cancers of the urinary tract, renal cell carcinoma (RCC) identified by urine cytology is extremely rare. CASE: A 74-year-old woman presented with painless hematuria. Urine cytology revealed cohesive clusters and single round-to-ovoid tumor cells with distinct cytoplasmic borders. Their nuclei were markedly enlarged and round-oval with uniform regular nuclear membranes, fine evenly distributed chromatin, and prominent macronucleoli. The cytoplasm was granular with occasional intracytoplasmic vacuoles and irregular cytoplasmic strands. Urine fluorescence in situ hybridization (FISH) revealed abnormal cells exhibiting polysomy for chromosomes 3, 7, and 17. No homozygous chromosome 9p21 loss was observed. Nephrectomy showed a sarcomatoid RCC with renal pelvis involvement. CONCLUSION: RCC identified by urine cytology was extremely rare, comprising only 2.4% and 0.003% of RCC and urine specimens, respectively. UroVysion FISH, a molecular-based assay for detecting malignant urothelial cells, was positive in 40% of our RCCs tested. FISH also managed to detect low-grade RCC as well as tumors without renal pelvis involvement. Thus, a positive UroVysion FISH test does not always assure a diagnosis of urothelial carcinoma and must be interpreted with caution in the presence of a renal mass. © Science Printers and Publishers, Inc.