We report a case of the early diagnosis of an arachnoid cyst. Transvaginal ultrasound examination of the fetal brain at 13 weeks' gestation revealed the presence of an anechoic mass above the posterior fossa. After extensive counseling and because of the uncertain prognosis of a first-trimester diagnosis of an arachnoid cyst, the parents opted for termination of pregnancy. The diagnosis was confirmed by postmortem examination.