A 33-year-old woman, gravida 2, para 1, was found to have a huge intra-abdominal cyst (2619 ml) in the 15th week. The cyst increased in volume to 3660 ml during the next 31 days. A tentative diagnosis of ovarian or pancreatic mucinous cystadenoma was made. Since the cyst was considered to cause fetal intra-uterine growth restriction, the patient underwent surgery in the 23rd week of pregnancy. The cyst was found to originate from the pancreas, and distal pancreatectomy with splenectomy was performed. The specimen weighed 5500 gm, which is the largest so far reported. Her postoperative course was uneventful, and she had a healthy full-term infant. Histological diagnosis was a benign mucinous cystadenoma. Up to the present, there have been three reported cases of pancreatic mucinous cystadenoma, including our case, and two cases of pancreatic mucinous cystadenocarcinoma, in association with pregnancy. Our case is the third reported of successful resection of the tumor during pregnancy resulting in a healthy infant.