Threatened miscarriage is a common problem during pregnancy.The aim of this prospective, open, randomised study was to determine whether dydrogesterone was more effective than conservative management alone in preventing miscarriage in women with vaginal bleeding up to week 16 of pregnancy. Women were excluded if they had a history of recurrent miscarriage. A total of 191 women were randomised to dydrogesterone (40 mg stat followed by 10mg twice daily) or conservative management (control group). The treatment was considered successful if the pregnancy continued beyond 20 weeks of gestation.The success rate in the dydrogesterone group was statistically significantly higher than that in the control group (87.5% vs. 71.6%; p<0.05). Miscarriage occurred in 12.5% of women in the dydrogesterone group compared with 28.4% in the control group (p<0.05). There were no differences between the groups with regard to the incidence of Caesarean section, placenta praevia, antepartum haemorrhage, preterm labour (weeks 28-36), pregnancy-induced hypertension or low birth weight (<2500 g) babies. There were no intrauterine deaths or congenital abnormalities in either group.Compared with conservative management, dydrogesterone had beneficial effects on maintaining pregnancy in women with threatened miscarriage.