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 karyotypes, there was karyotypic correspondence between villous cells at diagnosis and fetal fibroblasts. References 1. Benn P, Hsu L YF, Perlis T, Schonhaut A (1984): Prenatal diagnosis of chromosome mosaicism. Prenat Diagn 4: 1-9 2. Binkert F, Schmid W (1977) Pre-implantation embryos of Chinese hamster. I. Incidence ofkaryo- type anomalies in 226 control embryos. Mutat Res 46: 63-76 3. Boue J, Nicolas H, Barichard F, Boue A (1979) Le clonage des cellules du liquide amniotique, aide dans l'interpretation des mosaiques chromosomiques en diagnostic prenatal. Ann Genet 22:3-9 4.Hahnemann N (1973) Chromosome studies in induced abortions. Clin Genet 4: 328-332 5. Kalousek DK, Dill FJ (1983) Chromosomal mosaicism confined to the placenta in human con- ceptions. Science 221 : 665-667 6. Loft A, Tabor A (1984) Discordance between prenatal cytogenetic diagnosis and outcome of pregnancy. Prenat Diagn 4: 51-59 7. Markert CL, Petters RM (1978) Manufactured hexaparental mice show that adults are derived from three embryonic cells.