The incidence of chromosome mosaicism was analysed in 2612 consecutive chorionic villus sampling (CVS) cases. True mosaicism was found in five cases (0.19 per cent). Confined placental mosaicism (CPM) occurred in 51 cases (1.9 per cent). The chromosomes most frequently encountered in CPM were 2, 7, 8, and the sex chromosomes. Three types of CPM as proposed by Kalousek et al. (1992) were observed. Cases with CPM had a statistically significant association with spontaneous abortion (chi 2 = 3.92, p < 0.05). Based on the data analysed, we conclude that (1) CPM is as likely to occur in cultured chorionic villi as in the direct preparation; (2) as with amniocyte culture, chromosome 2 mosaicism occurs commonly in villus culture; (3) the incidence of false-positive results can be greatly reduced and concern about maternal cell contamination can be alleviated by using both direct cytotrophoblast preparation and cultured chorionic stroma cells in all cases; and (4) follow-up cytogenetic and obstetrical studies, such as ultrasound and fetal monitoring, should always be considered in cases where CPM is suspected.