Objective To test whether supervised pelvic floor exercises antenatally will reduce the incidence of postpartum stress incontinence in at-risk primigravidae with bladder neck mobility, ultrasonically proven. Design Single blind, randomised controlled trial.Setting Antenatal clinic in a UK NHS Trust Hospital.Sample Two hundred and sixty-eight primigravidae attending an antenatal clinic at approximately 20 weeks of gestation with bladder neck mobility, on standardised valsalva, of 5mm or more linear movement. The median age was 28, ranging from 16 to 47 years. Intervention Patients randomised to supervised pelvic floor exercises (n ¼ 139) attended a physiotherapist at monthly intervals from 20 weeks until delivery. The exercises comprised three repetitions of eight contractions each held for six seconds, with two minutes rest between repetitions. These were repeated twice daily. At 34 weeks of gestation the number of contractions per repetition was increased to 12. Both the untreated control group and the study group received verbal advice on pelvic floor exercises from their midwives antenatally. Main outcome measures Subjective reporting of stress incontinence at three months postpartum. Pelvic floor strength, using perineometry, and bladder neck mobility measured by perineal ultrasound. Results Of the 268 women enrolled, information on the main outcome variable was available for 110 in the control group and 120 in the study group. Fewer women in the supervised pelvic floor exercise group reported postpartum stress incontinence, 19.2% compared with 32.7% in the control group ). There was no change in bladder neck mobility and no difference in pelvic floor strength between groups after exercise, although all those developing postpartum stress incontinence had significantly poorer perineometry scores than those who were continent. Conclusions The findings suggest that antenatal supervised pelvic floor exercises are effective in reducing the risk of postpartum stress incontinence in primigravidae with bladder neck mobility.