The activity and side-effects of the prostaglandin derivatives, sulprostone and gemeprost were compared, 188 patients were studied, 90 receiving sulprostone 500 micrograms intramuscularly 16 hours pre-operatively and 98 receiving gemeprost 1 mg intravaginally 4 hours pre-operatively. Dilatation of the cervix of more than Hegar 8 was obtained in 80 per cent of the sulprostone group and in about 50 per cent in the gemeprost group. On dilatation before vacuum aspiration, little or no resistance was detected in nearly 80 per cent of both groups. Side-effects occurred in 39.3 per cent of the sulprostone group (usually abdominal pain), and in 22.6 per cent of the gemeprost group and here again abdominal pain was predominant. The results showed that with both sulprostone and gemeprost satisfactory dilatation of the cervix could be obtained. The somewhat better results with sulprostone are obtained at the expense of a higher incidence of side-effects. The administration of gemeprost was also better tolerated by the patients.