At 17th week of pregnancy, a 28-year-old woman was diagnosed as having herpes simplex encephalitis and treated with intravenous acyclovir. Follow-up by the serial ultrasound examinations, intrauterine growth retardation (IUGR) was found. During the course of disease, cordocentesis was performed to evaluate the risk of the disease and the infant's chromosomal constitution. No herpes simplex virus infection on cord blood sample was observed; however, chromosomal analysis revealed: 46,XX/47,XX,+2/47,XX,+11/47,XX,+19/48,XX,+11. After termination of pregnancy, the fetus was found as having ventricular septum defect. The presence of the triploid cell lines mocaicism involving chromosome 2 and 19 were confirmed by the analysis of fetal skin tissues. No attributable finding to herpes simplex virus infection and acyclovir treatment was found, and the presence of the triploid cell lines mocaicism were appeared to be purely coincidental.