Renal cell carcinoma (RCC) is the most common form of kidney cancer in adults. The gross and light microscopic appearance of RCC's are quite variable, but cases can be diagnosed reliably on the basis of the 2004 World Health Organization classification system, which defines tumor subtypes with distinctive light microscopic morphology, clinical behavior and underlying genetic mutations.

Light microscopic morphology-based diagnosis of RCC subtypes can be difficult due to variable histologic and morphologic features shared by tumor subtypes. Approximately 7% of RCC's are diagnosed as unclassified RCC (uRCC), owing to the presence of overlapping oncocytic and sarcomatoid features that can be seen in the subtypes of RCC, unusual cellular morphology and growth pattern that has not been described in known subtypes of RCC or owing to the presence of combined epithelial and stromal morphologies.

The term uRCC is useful because we avoid a forced use of the well-defined cellular subtypes but a careful pathological study should be performed before taking this decision. Further molecular and genetic studies will define exact nature and prognosis of uRCC. The aim of this review is to describe and identify uRCC in pathological practice and to provide suggestions in clinical practice.