A 59-year-old man with spinal arteriovenous malformation at L-1 level was referred to our clinic for urinary retardation and urinary protraction. Bulbocavernosus reflex (BCR) was performed at empty bladder and at the storage phase during cystometry. BCR was evoked by compressing the glans penis. The evoked reflexes were examined by palpating the contractile responses of the bulbocavernosus muscle and bipolar surface electrodes on each side of para-perineal raphe. Evoked response of BCR couldn't be detected at empty bladder, but obvious responses of BCR were detected at about 300ml bladder filled. For further evaluation, a concentric needle electrode was inserted into the periurethral striated muscle to examine the evoked potential of BCR. The responses were unstable at empty bladder, but stable evoked potentials were obtained at storage phase. BCR performed only at empty bladder may cause false negative result.