In 3,000 chorionic villi studies (CVS) 33 cases of mosaicism and 7 false-positive cell lines in all cells were seen. The mosaic cell lines were caused by aneuploidy of autosomes (13x), sex chromosomes (9x), and structural anomalies (11x). Mosaics of fetal origin were only 4 cases of trisomy 21 and one 47,XXY mosaic. In 7 cases abnormal karyotype of non-fetal origin was seen in all cells in direct studies, including trisomy 16 (3x) and trisomy 18 (2x). The combined use of direct CVS and cell cultures always uncovered the non-fetal origin of chromosome abnormalities and the study of cultured cells in all cases could have prevented 5 terminations. Complete follow-up studies demonstrated no false-negative results. Therefore, CVS can be nearly 100% accurate when both direct studies and cultures are examined in cases of mosaicism and other cell lines of possible non-fetal origin, such as trisomy 16, trisomy 18, translocation (21;21), and 45,X cells.