Before the introduction of ultrasonography (US), ovarian cysts in newborns were thought to be rare. With the extended use of real-time US, prenatal detection has increased. There is still considerable controversy regarding the best treatment of these neonatal findings. A total of 27 instances in 24 consecutive newborns of antenatally-diagnosed ovarian cysts were reviewed for US data, management, and outcome to assess the ante- and postnatal evolution of the cyst and establish appropriate therapy. The cysts detected during pregnancy were studied by repeated postnatal US studies. In 10 children (2 with bilateral cysts) the cyst evolved spontaneously toward regression. Fourteen patients were operated upon; 10 had complex and 4 (in 1 bilateral) had simple cysts. Cystectomy was employed when possible to preserve the ovarian parenchyma (3 cases); in the remaining cases oophorectomy was performed. Histologically, the cysts were of follicular origin or necrosis made a diagnosis impossible. US was found to be a helpful diagnostic tool for simple ovarian cysts, but could not reliably distinguish between benign and malignant tumors if a sonographically complex lesion was present. Based on our experience, surgical management of ovarian cysts should be reserved to complex masses. Simple cysts can be monitored safely by close US follow-up; surgery is indicated if the cyst fails to regress after several months or becomes symptomatic.