The authors report over 100 sacral root blocks performed in 50 patients with detrusor activation by air cystometry. The detrusor reflex was abolished with unilateral sacral blocks in over 50% of the patients. The nerve roots most frequently innervating the bladder were S 3 and S 4. Most of these patients suffered from multiple sclerosis and had spinal cord involvement. The possible pathophysiology and its significance regarding urinary bladder innervation are discussed.