Endometriosis in infancy is most unusual, and associated tumors in this age group are exceptionally rare. We report a case of a serous borderline tumor and endometrial stromal sarcoma arising in an ovarian endometriotic cyst. The patient was an infant of 18 months of age who presented with an incidental abdominal mass. The serum sex hormones were at prepubertal levels. There was no evidence of precocious puberty or any obvious genital anomaly. Intraoperative findings included a solitary solid and multicystic right ovarian mass without evidence of any extraovarian disease. On microscopic examination, the tumor was composed of an intimate mixture of florid papillary and stromal cell proliferation in the wall of an endometriotic cyst. The papillae showed hierarchical branching and had hyalinized and edematous cores with scattered psammoma bodies. The epithelial cells were mildly atypical and mitotically inactive. The underlying endometrial stromal cells were arranged in irregular tongues that permeated the thickened fibrous cyst wall. They were mitotically active and immunoreactive for CD10. There was no evidence of any primitive germ cell tumor. The patient received no adjuvant treatment and had an uneventful postoperative follow-up period of 30 months. To the best of our knowledge, endometriosis associated with this most unusual combination of ovarian tumors has never been reported in an infant.