Objectives: In this study, the aims were to reveal the incidence of isolated choroid plexus cyst in our population, and to discuss the accuracy of distinguishing either an isolated or non-isolated choroid plexus cyst. Methods: The study population was consisted of 10 594 pregnant women. The patients with choroid plexus cysts were classified into two groups: isolated and non-isolated. Detailed ultrasonographic examination and genetic counseling were performed and triple screening test was ordered. The incidence, sensitivity, specificity, false-positive rate and likelihood ratio of cases with isolated choroid plexus cyst for trisomy 18 were determined. Results: Choroid plexus cysts were identified in 109 patients (109/10 594; 1.02%). In 102 patients isolated choroid plexus cysts, and in seven patients additional fetal anomalies supporting trisomy 18 were detected. Trisomy 18 was detected in four patients, and one of them had isolated choroid plexus cyst. The likelihood ratio in cases of isolated choroid plexus cysts for trisomy 18 was 9.51 (95% confidence interval, 0.2-41). Conclusions: According to the study the individual risk for trisomy 18 in isolated choroid plexus cyst should be calculated by using the likelihood ratio. These data allows the physician to express the individual risk of trisomy 18 and permits more accurate genetic counseling. (C) 2003 International Federation of Gynecology and Obstetrics. Published by Elsevier Ireland Ltd. All rights reserved.