Of 646 cases involving first trimester chromosome analysis of chorionic villi sam ples obtained by trans cervical aspiration, we found 34 abnormal unbalanced karyo types. In six of these, fetal fibroblast cultures obtained after termination of the preg nancy failed to confirm the abnormality. Three cases were mosaics identified from direct preparations: 46, XY/45, X; 46, XX/47, XX, +3; and 46, XY/47, XY, +18. In two cases of trisomy 16 (46, XY/47, XY, +16; 46, XX/47, XX, +16) and one oftriso my18 (46, XX/47, XX, +18), no mosaicism was detected. In the other 28 cases with unbalanced abnormal...