Primary tumors of the renal pelvis and ureter account for about 8% of all urinary tract tumors. More than 90% of them are urothelial carcinomas. On the other hand, unilateral multicystic renal disease is an uncommon pathologic condition that may be mistaken for unilateral autosomal dominant polycystic kidney disease, multilocular cystic nephroma or cystic neoplasm. We present the case of a 54-year-old male known with arterial hypertension, admitted in the Second Surgery Department of Emergency County Hospital, Constanta, with intense right flank and right lumbar pain. This symptom started one month before hospital admission. Based on clinical features and imaging evaluations we established a presumptive diagnosis of unilateral autosomal dominant polycystic kidney disease. For these reasons, total right nephrectomy was performed. Pathologic examination of the nephrectomy specimen revealed high-grade urothelial carcinoma of the renal pelvis associated with unilateral multicystic renal disease. The particularity of this case lies in the uncommon association between two rare renal pathological conditions diagnosed by pathological examination.