We report the ultrasound, cytogenetic and morphologic findings in a case of trisomy 10 mosaicism prenatally detected by chorionic villus sampling (CVS). CVS sampling was carried out at the 13th week of gestation because of ultrasound diagnosis of hydrops fetalis and hygroma colli. Trisomy 10 mosaicism was diagnosed in cells from the cytotrophoblast (short-term culture) and the chorionic villus core (long-term culture). Fetal mosaicism was confirmed after termination of pregnancy in umbilical cord cells, placenta and fetal skin fibroblasts.