This study investigated 29 nulliparous women using the technique of transvaginal ultrasound to assess whether their bladder necks were open or closed at rest. The patients comprised 2 groups: 4 reported occasional episodes of stress incontinence, all of whom had closed bladder necks; the remaining 25 patients were totally asymptomatic. Overall a 21% incidence of an open bladder neck was recorded. It is likely that the true incidence of open bladder necks in young nulliparous women is higher than this, since none of these patients had troublesome stress incontinence. Since women with open bladder necks are more likely to develop stress incontinence if the integrity of the distal sphincter mechanism is compromised by neural damage, antenatal recognition of this problem should provide a contraindication to traumatic vaginal delivery and may in the future reduce the incidence of symptomatic stress incontinence in the population.