We report a case of a 65-year-old man examined for hematuria and neurologic symptoms. Radiologically a tumour of the kidney was suspected, with metastatic involvement of the brain and both lungs. The urologists then performed a cytoreductive nephrectomy.In histopathological examination of the nephrectomy specimen a clear cell renal cell carcinoma (clear cell RCC) was found. However, an additional focus of a high-grade adenocarcinoma was discovered that turned out to be a metastasis of a poor differentiated lung adenocarcinoma. This diagnosis was supported by positive TTF1 and napsin-A as well as good clinical correlations.The article summarizes the differential diagnostic considerations of poor differentiated adenocarcinoma of the kidney, aimed at gross and microscopic morphology, immunohistochemistry and clinical-pathological correlation. A review of literature about a tumour-in-tumour metastasis is included.