Struma ovarii is a teratoma composed of thyroid tissue and can be a rare cause of hyperthyroidism. We report a 35 years old woman with a left ovarian mass whose pathology revealed a Struma ovarii. This tumor was partially excised initially to avoid fertility problems and after a successful gestation, the tumor was completely removed during a cesarean section. Sixty days after delivery a hypothyroidism was diagnosed. Levothyroxine was started and euthyroidism was achieved (Rev Méd Chile 2003; 131: 785-7)