We report here the case of a 30-year-old woman with a decidualized ovarian endometriotic cyst (DOEC) during pregnancy mimicking malignancy occurring after fertility-preserving surgery for ovarian carcinoma arising from an endometriotic cyst. Intracystic excrescences appeared in the left ovarian endometriotic cyst at five weeks and three days of gestation. The serum CA-125 level rose to 676.7 U/ml (normal, 0-35). Based on these findings, ovarian carcinoma arising from the left ovarian cyst was strongly suspected. Frequent sonographic examinations revealed that the sizes and quality of the intracystic excrescences remained essentially unchanged. The cyst was evaluated for DOEC during pregnancy. The patient eventually delivered a male infant by normal vaginal delivery. After the delivery, the intracystic excrescences in the left ovarian endometriotic cyst disappeared. Close observation may be a reasonable alternative to antepartum surgery in patients with a DOEC during pregnancy.