Authors report clinicopathological features of an unusual case of composite renal lesion occuring in 32-year-old Caucasian male. The patient was followed for cystic lesion of retroperitoneal-renal region for 5 years. He was indicated for resection of the cystic lesion because of changes of the retroperitoneal mass on CT scan. A cyst was located on upper renal pole. A huge cystic mass filled mainly by necrotic material was resected and submitted for histological examination. The wall of the cyst was composed of fibrous tissue, indistinguishable from inflammatory pseudotumor on histological level. The vital intracystic tissue was formed by well-differentiated papillary renal cell carcinoma. The most important step within differential diagnosis is distinguishing of sarcomatoid differentiation in renal cell carcinoma. This very rare case demonstrates the importance of careful examination of all spindle cell lesions of the kidney.

        Key words: kidney – papillary renal cell carcinoma – fibrosis – inflammatory pseudotumour