25 pregnant women, 15 of whom developed stress incontinence in pregnancy, have been investigated by cystometry and urethrometry in both erect and supine posture. The resting bladder pressure rises throughout pregnancy in both erect and supine positions. Those who are not incontinent have normal bladder and urethral function. Incontinent patients show no residual urine, normal bladder capacity, and either detrusor instability and/or sphincter weakness. The stress of change of posture makes both bladder dysfunction and sphincter weakness more evident. Although the results of this investigation are not yet complete, there is a suggestion that detrusor instability may be produced by pregnancy in some patients and that in some instances recovery to normal bladder function occurs after delivery. It is postulated that these patients may be the ones who, in later life, develop stress incontinence due to bladder dysfunction.