Treatment of idiopathic detrusor instability with drugs and prolonged distension of the bladder is often unsatisfactory. Some amelioration of symptoms was previously described after 24-hour bladder distension with use of the cystomat. A case is presented in which detrusor instability was successfully treated with continuous infusion via the cystomat, and thereby repeated bladder distension, for one week. The symptoms subsided, the cystometric bladder capacity was increased and there was no side effects. The improvement has persisted for six months. Though the mechanism by which the bladder distension exerts its effect is poorly understood, the technique of repeated distension offers an alternative in the treatment of patients with idiopathic detrusor instability.