152 women admitted for legal abortion in the second trimester of pregnancy were included in an open, randomized, controlled six-centre study. 75 patients received gemeprost 1 mg vaginal pessaries at 3 hours intervals up to a maximum of 5 mg and 66 patients were treated with a single 40 mg intra-amniotic dose of PGF2 alpha. The 24-hour success rate was 81% (n = 61/75) in the gemeprost and 64% (n = 42/66) in the PGF2 alpha group (p less than 0.02). The mean abortion times were 14.3 and 14.8 hours in the gemeprost and the PGF2 alpha groups, respectively. The mean time to onset of pain was shorter and more patients experienced blood loss over 100 ml during the induction in the PGF2 alpha group than in the gemeprost group (p less than 0.02). Apart from that, the nature and severity of side effects were comparable between the two groups. Besides significantly better efficacy, the non-invasive gemeprost treatment was found to be easier and safer as compared to the PGF2 alpha treatment.