Background: Perinatal findings of abdominal masses pose a diagnostic challenge to clinicians. This study presents the operative findings of patients who underwent exploration for perinatally identified abdominal masses of unknown etiology. Methods: Retrospective review of all patients with abdominal masses of unknown etiology identified in the antenatal period was conducted from January 1, 2000 to July 1, 2010. Patient demographics were collected. Preoperative radiographic studies, operative findings, and pathologic evaluation were reviewed. Results: There were 17 patients identified within the study period. The median age was 30 d at the time of operation (range 0-287 d). The median height was 51 cm (range 45-77 cm), and the median weight was 4.0 kg (range 2.6-10.4 kg). All patients were asymptomatic. After birth, ultrasound identified abdominal masses in 14 patients, and computed tomography scan was used in four patients where one patient had both an ultrasound and a computed tomography scan. Mass resection was performed using laparoscopy in 15 patients, whereas two patients underwent open resection. At the time of surgery, 11 patients were diagnosed with ovarian cysts, four patients with ovarian torsion with an associated cyst, and two patients with an enteric duplication cyst. On final pathology, eight patients had benign ovarian cysts, seven patients had hemorrhagic ovarian necrosis, and two patients had duplication cysts. Conclusion: Females with antenatally identified abdominal masses of unknown etiology appear to be benign in nature. In this series, a benign ovarian cyst is the most common diagnosis, and these lesions can be approached laparoscopically. © 2012 Elsevier Inc. All rights reserved.