This paper describes seven cases of confined chorionic mosaicism with trisomy 3. The chromosomally abnormal cell line in chorionic villi was revealed in three cases at diagnostic CVS and in four cases at the evacuation of the uterine cavity after a missed abortion had been diagnosed by ultrasound. In two of these cases, the abortion occurred after apparently normal development of the fetus during the second trimester of pregnancy. An evaluation of the effect of confined chorionic mosaicism with trisomy 3 on the viability of the conceptus has been attempted.