BACKGROUND Morphologic analysis of malignant renal tumors of childhood and adolescence has resulted in the identification of a variety of tumor types with characteristic histology and clinical behavior. The authors report a case of renal sarcoma in a 19-year-old male that differs in morphology from the various established categories of primitive renal tumors. METHODS Sections taken from the nephrectomy specimen were stained by routine methods and by immunohistochemistry for stromal and epithelial markers, and for proliferation markers. In addition, ultrastructural studies were undertaken. RESULTS The tumor, which the authors termed cystic embryonal sarcoma, was comprised of poorly differentiated malignant mesenchyme in a myxoid stroma. Numerous epithelial-lined cysts were present. The distribution of the cysts and proliferation kinetics of cyst-lining epithelial cells suggested that they were derived from entrapped renal tubules. The tumor showed early recurrence postoperatively and after aggressive chemotherapy. The pathologic features and clinical behavior of the tumor resemble those of 2 previously reported cases and an additional 25 cases from the files of the National Wilms' Tumor Study Pathology Center. CONCLUSIONS The clinical and histologic features of cystic embryonal sarcoma differ from those of other renal tumors of childhood and adolescence, and the tumor appears to be a novel form of renal malignancy.