Two cases of simultaneously developed renal tumors (renal cell carcinoma and renal pelvic urothelial carcinoma) in the ipsilateral kidney are reported. These underwent radical surgery under the diagnosis of renal cell carcinoma or renal pelvic urothelial carcinoma, respectively. The risk of incorrect preoperative diagnosis is discussed in terms of insufficient surgical margins. The liberal use of frozen section diagnosis and more radical resection if applicable may be the key to solve such dilemmas.