We describe a patient with bilateral cystic tumors of the pelvis. The left one rapidly grew during pregnancy and combined with the right one, whose clinical course made diagnosis difficult. A pregnant woman with a history of laparotomy was referred to us due to suspected bilateral pelvic cysts. The left-sided cyst had rapidly grown to 27 cm in diameter and merged with the right cyst, forming a large cyst occupying the entire pelvic cavity in the third trimester. Considering this rapid growth, cesarean section and resection of the cyst were performed at 37th week. The resected cyst consisted of two components: a large unilocular cyst containing serous fluid and a multilocular cyst suggestive of ovarian mucinous cystadenoma in the right ovary. The wall of the former largely lacked lining epithelium, but it was partly continuous with the latter mucinous epithelium. Immunohistochemically, estrogen and progesterone receptors were focally positive in the cyst wall, suggesting that pregnancy-associated sex-hormones may have contributed to the rapid growth of the cyst. We diagnosed this condition as a peritoneal inclusion cyst margining with a right ovarian mucinous cystadenoma. Peritoneal inclusion cyst should be considered in the differential diagnosis of a rapidly growing pelvic mass during pregnancy.