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 cel