Objective : To assess the clinical significance of isolated enlargement of the cisterna magna and to help obstetrician in further management. Methods : All prenatal sonography was reviewed, which was performed from 1996 to 1999. Prenatal sonographic findings and medical records of pregnant women and their babies were analyzed in 15 cases with regular antenatal care and delivery results at Severance hospital, out of 25 cases with isolated enlargement of the cisterna magna. Results : The mean maternal age was 29.2 years and the mean gestational weeks at diagnosis was 30 weeks. 5 cases were diagnosed during the 2nd trimester, 10 cases during the 3rd trimester. 13 cases were found before 32 gestational weeks. Prenatal cytogenetic study was made in 2 cases, of which results were normal. 12 fetuses were male and 3 female, showing male predominance. APGAR score after birth was good and phenotypic evidence of aneuploidy was not observed in all cases. Postnatal follow-up could be done in 7 cases, during which delayed development and neurological abnormality was not found. Conclusion : Our data demonstrate that it is important to assess other findings of chromosomal abnormality by performing detailed prenatal ultrasonography and that prenatal cytogenetic study and postnatal follow-up may be unnecessary in cases with isolated enlargement of the cisterna magna.