Adnexal masses affect 2-10% of all pregnancies. The highest incidence of 1-6% can be seen in the first trimester, with a high rate of spontaneous remission. Two percent of these masses are malignant neoplasms or borderline tumors. A rare benign mass of the adnexa in pregnancy is hyperreactio luteinalis characterised by bilateral, multicystic ovaries with a frequent occurrence in the 3rd trimester. Clinical signs include maternal hyperandrogenaemia with virilisation, hyperemesis, nonspecific abdominal pain or laboratory findings of hyperthyroidism and elevated ß-HCG. Hyperreactio luteinalis does not require therapy due to complete spontaneous remission postpartum, but is often treated surgically in graviditate. In our case we report a first-time gravida in the 31st week of pregnancy with a symptomatic 25-cm multicystic, partly solid mass. After antenatal corticosteroid therapy, an exploratory laparotomy with right adnexectomy was performed on suspicion of malignancy. Histology revealed a hyperreactio luteinalis with an incidental finding of a serous borderline tumor of the ovary (FIGO IIIB). At 33 weeks of gestation, a pathological CTG was observed, and an urgent secondary caesarean section by re-longitudinal laparotomy was performed. Postpartum completion surgery revealed no further neoplastic cells.