ObjectiveTo determine the optimal dosage of vaginal misoprostol for cervical priming before vacuum aspiration abortion.MethodsOne hundred twenty women were assigned randomly to receive 200, 400, 600, or 800 μg of misoprostol given vaginally. Vacuum aspiration was performed 3–4 hours after the insertion of misoprostol tablets. The degree of cervical dilation before operation was measured with a Hegar dilator. Preoperative and intraoperative blood loss and associated side effects also were assessed.ResultsTwenty-nine (96.7%) women in the 400-μg group and all in the 600-μg and 800-μg groups achieved cervical dilation of at least 8 mm. The success rate for the 200-μg group was only 23.3%, significantly less efficacious than the 400-μg dose (odds ratio 95.3; 95% confidence interval 10.9, 830.9;P< .001). There was no significant difference among the 400-, 600-, and 800-μg groups (P= .364) with respect to achieving cervical dilation at least 8 mm. However, 800 μg was associated with significantly more side effects than 600 μg (preoperative and intraoperative blood loss,P< .001; abdominal pain,P= .005; products of conception at os,P< .001; fever higher than 38.0C,P= .002). When 400 μg and 600 μg were compared, we found that the higher dose also was associated with significantly more side effects. The 600-μg group was used twice in the comparison, but allPvalues remained significant even after the Bonferroni adjustment for multiple comparisons.ConclusionVaginal application of 400 μg of misoprostol is the optimal dose for vacuum aspiration preabortion cervical dilation in first-trimester nulliparas.