SummarySoft ventouse cups are becoming increasingly popular for assisted vaginal delivery. We report the results of a prospective random allocation controlled trial comparing two such cups. The results suggest that the silicone rubber cup is more likely to achieve delivery than a similarly designed cup made from santoprene [odds ratio 4.65, 95 per cent confidence interval 1.3 to 18.8]. Consequently we recommend the silicone cup for delivery when a soft ventouse cup is indicated.