We describe a retroperitoneal neoplasm in an 11-year-old girl which had a light microscopic appearance identical to that of papillary serous carcinoma of the ovary. There was no evidence of ovarian involvement. Immunohistochemical staining for amylase was positive within the cytoplasm of tumor cells. Since amylase is a marker for serous ovarian tumors, this finding supports the belief that "ovarian-type" neoplasms that occur at ectopic locations are essentially identical to their ovarian counterparts. We believe they originate from metaplasia of mesothelium. Our findings support the concept that these tumors should exhibit a biologic behavior and therapeutic response which are similar to those of an ovarian tumor of the same grade and comparable stage. The demonstration of intracytoplasmic amylase also may prove useful in differentiating peritoneal serous tumors from non-metaplastic mesothelial proliferations. We are unaware of a prior report of an extra-ovarian serous carcinoma in a child.