The effect of a prostaglandin E1 analogue on the cervix prior to termination of first trimester pregnancy was studied in a double-blind placebo-controlled trial with 70 primigravid and 125 multigravid patients in the Hong Kong University Gynaecological Unit. One mg of 16, 16-dimethyl-trans-delta 2-prostaglandin E1 methyl ester (ONO 802) in the form of vaginal pessary was inserted 3 hours before vacuum aspiration. The cervical dilatation in patients receiving ONO 802 pessaries was significantly greater than that in patients who received no treatment or placebo pessaries but the difference in dilatation between patients who received no treatment or placebo was not significant. The blood loss during operation in patients receiving ONO 802 pessaries was also significantly less than that in patients receiving placebo pessaries. The side effects due to ONO 802 pessaries were mild and infrequent.