We report a case of ovarian cystic teratoma with an important growth during pregnancy and the sonographic appearance of intracystic multiple, mobile, polygonal structures called intracystic "fat balls." Due to the rapid growth of the lesion, which exceeded 15 cm in diameter, a right oophorectomy was performed. Histopathologic diagnosis confirmed the mature cystic teratoma. The presence of floating balls composed of keratin and fat is rarely seen but is pathognomonic of mature cystic teratomas. Growth of a teratoma during pregnancy is a rare condition.