Objective: This study was undertaken to compare the effectiveness of mifepristone orally administered at 24 or 48 hours before first-trimester vacuum aspiration abortion with that of vaginally administered misoprostol as a cervical priming agent. Study Design: In a randomized comparative trial 90 women who requested surgical termination of pregnancy were randomly assigned to receive 200 mg mifepristone orally 24 or 48 hours before the operation or 800 μg misoprostol vaginally 2 to 4 hours before the operation. The main outcome measures were baseline cervical dilatation, cumulative force required to dilate the cervix to 9 mm, and intraoperative blood loss. Results: The baseline cervical dilatation was significantly greater among women who received mifepristone 48 hours before the operation (P = .02). This group also required the least mechanical force to dilate the cervix (P = .06). There were no significant differences among the 3 groups in the intraoperative blood loss, in the operating time, or in patient acceptability. Side effects such as hot flushes and headaches were significantly higher among women who received mifepristone 24 or 48 hours before the operation than among those who received misoprostol (P = .01 and P = .002, respectively). Conclusion: Mifepristone is an effective cervical priming agent when orally administered 48 hours before vacuum aspiration for termination of first-trimester pregnancy. Because of its cost and availability in comparison with misoprostol, however, selective use may have to be considered. (Am J Obstet Gynecol 2000;183:998-1002.)