In 458 consecutive chorionic villus sampling (CVS) procedures, we observed a significant influence of uterine position upon sampling efficacy. Compared with anteverted (N= 243) or axial (N= 149) locations, the retroverted uterus (N= 66) was associated with a lower mean sample weight per aspiration (22, 18, and 15 mg, respectively;P< .01) and a greater frequency of multiple-pass procedures (23, 31, and 52%, respectively;P< .0001). To improve sampling efficiency in selected cases of uterine retroversion, we adopted a transvesical approach. When compared with transabdominal or transcervical techniques, transvesical CVS had the highest single-pass success rate (33,33, and 60%, respectively). Only one in 30 transvesical cases required three placental passes, compared with nine of 36 retroverted uteri sampled by either transabdominal or transcervical techniques (P< .05). The mean transvesical sample weight was 18.7 mg; at least 10 mg was retrieved in all cases. Post-procedure bleeding occurred in four instances and an additional patient suffered a spontaneous loss at 16 weeks' gestation. Aneuploidy was found in four of 30 biopsy specimens, and the remaining pregnancies either have delivered at term (N= 18) or are continuing (N= 7). Our preliminary experience suggests that selected use of this CVS method may improve sampling efficiency without increasing the incidence of complications.