OBJECTIVE To tabulate genetic results and obstetric outcomes of twin pregnancies after first-trimester chorionic villus sampling (CVS). METHODS The study included 262 consecutive women with twin pregnancies who had first-trimester CVS between 1988 and 1998. RESULTS Major indications for prenatal diagnosis included maternal age (n = 82), pregnancies after intracytoplasmic sperm injection (n = 114), or both (n = 33). Among 524 fetuses, 519 were sampled adequately. Cytogenetic results were incorrect because of sampling the same fetus twice in two pregnancies. In three pregnancies, contamination caused by mixed sampling made cytogenetic results uncertain. Correct genetic diagnoses were obtained in 509 fetuses, 24 of which had chromosomal abnormalities on direct preparations and four of which had monogenetic conditions. Additional invasive procedures were done on five occasions. Fifteen fetuses were terminated selectively. The total fetal loss rate was 5.5% (28 of 509). The indication for the procedure did not significantly determine the fetal loss rate. The mean +/- standard deviation (SD) gestational age at birth was 35.9 +/- 2.9 weeks, and the mean +/- SD birth weights for twins A and B were 2429 +/- 589.1 g and 2378 +/- 588.5 g, respectively. CONCLUSIONS First-trimester CVS is an accurate means of prenatal genetic diagnosis in twins, offering early selective termination in cases of abnormal genetic results in one of the fetuses.