A multicentre trial was conducted to compare the efficacy and side-effects of an intramuscularly administered PGE2 analogue and vacuum aspiration in women with a delay of up to 21 days in the expected onset of menses. A total of 473 such women were randomly allocated to treatment with either 16-phenoxy-W-17, 18, 19, 20-tetranor PGE2 methyl sulfonylamide (three intramuscular injections of 0.5 mg at 3-h intervals) or vacuum aspiration, and the outcome of therapy assessed 1, 2 and 6-8 weeks later. Retrospective analysis of hCG levels indicated that 419 (88.6%) women had been pregnant at the time of treatment. With few exceptions, administration of the PGE2 analogue induced vaginal bleeding in both pregnant and non-pregnant women but the duration and subjectively perceived amount of bleeding were greater than after vacuum aspiration. Both treatments were equally effective. In pregnant women the overall frequency of complete abortion was 91% for prostaglandin treatment and 94% for vacuum aspirations. If non-pregnant women were included, the respective success rates (i.e. percentages of women not pregnant 2 weeks after treatment) were 92% and 95%. Gastrointestinal side-effects and lower abdominal pain requiring intramuscular analgesia were more common after prostaglandin therapy than following vacuum aspiration in both pregnant and non-pregnant women.