An 11-year-old girl with Down syndrome (DS) was operated for a stage I right ovary dysgerminoma. She is in good health 33 years later. Some data in the literature suggest that ovarian cancers could be slightly overrepresented in DS. Despite the rarity of ovarian dysgerminoma, our case is the fifth reported in DS. This case is the second one associated with a family history suggesting the possibility of a familial predisposition to cancer. A hypothesis explaining the development of dysgerminoma in DS is proposed.