Adult cystic nephroma (CN) and mixed epithelial and stromal tumor of the kidney (MEST) are considered as separate entities in the 2004 World Health Organization classification of renal neoplasms. Recent studies suggested that the two share clinicopathologic features and may represent the same disease process of varying morphology. However, definitive genetic evidence is lacking. We examined their relationship using gene expression profiling and histologic analysis. Gene expression profiles of 3 CN and 3 MEST were analyzed using HGU133 Plus 2.0 microarrays (Affymetrix) and were compared with each other and also with 48 other renal tumors and 13 normal kidneys. Histologic examination of 26 CN and 13 MEST focused on the cystic septal thickness, cyst-to-stroma ratio, stromal cellularity and composition, types of epithelial cells lining cysts and glands, and estrogen and progesterone receptors expression. Patients' age, sex distribution, and tumor size were similar between the two. They also shared many histologic features, including lining epithelium of cysts and glands, stromal cellularity and composition. Unsupervised clustering of mRNA expression profiles demonstrated that they had very similar expression profiles that were distinct from other renal tumors. By microarray analysis, progesterone receptor expression was significantly higher in CN and MEST relative to both normal and other renal tumors, while estrogen receptor expression was not. By immunohistochemistry, expression of both receptors was similar between CN and MEST. This study provides the most convincing molecular evidence that CN and MEST represent different parts of the morphologic spectrum of the same disease.