This paper reports eight cases of non-mosaic, rare, and typically lethal trisomies diagnosed in chorionic villi and not confirmed by amniocentesis. Four cases were 47,XX,+16; two cases were 47,XX,+2; one was 47,XX,+12; and one was 47,XY,+7. There have been no known complications in any of these gestations. These eight cases were found in a series of approximately 12,000 samples processed in our laboratory (0.07 per cent). We conclude that (1) rare non-mosaic trisomy not reflecting the fetal condition is an occasional source of diagnostic ambiguity in chorionic villus sampling; and (2) when encountered, a follow-up amniocentesis should be recommended to the patient to confirm or rule out the abnormality. We propose the term 'confined placental abnormality' to describe non-mosaic trisomies and other related abnormalities found only in chorionic tissue.