Thirty-four urodynamic studies were performed in 20 patients with spinal cord injury (SCI) to determine the most effective triggering mechanism for reflex voiding. The studies were performed at a time when the patient was normally scheduled for catheterization, which avoided stimulation of the detrusor and sphincter by bladder filling via a catheter. The three methods chosen were used in a random sequence. Suprapubic tapping and jabbing were equally effective in producing a rise in detrusor pressure, and the sphincter responses were almost identical. Cutaneous stimulation of the thigh rarely produced any change in detrusor and sphincter activity. When detrusor contractions were produced, a dyssynergic sphincter response prevented voiding in 46% of the studies; however, voiding always occurred when the sphincters were either coordinated or showed no change. Both tapping and jabbing were more effective as the time from injury increased, which reflects the natural recovery from spinal shock.