The authors report the results of chromosomal analyses performed on 6235 amniocenteses and 559 choriocenteses. Whereas the frequencies of chromosomal anomalies observed respectively on amniocenteses and choriocenteses did not differ significantly, the comparison of the types of aberrations found revealed, in chorion villi, a relatively high proportion of lethal anomalies, never encountered in amniocyte cultures. Furthermore, chromosomal mosaicism was observed 10 times more frequently on chorion villi than on amniotic cells. These results are globally comparable to those reported in other surveys. In view of literature reports of discordances between fetal chorionic karyotypes, never found in amniocenteses, rapid karyotyping from chorion villi is not as reliable as from amniotic cells. Taking into account the risk of cytogenetic discordance specific to choriocentesis, it is recommended that this method be strictly limited to pregnancies with high genetic risk.