Narrowing of the urethra in the region of the external sphincter as seen during voiding cystourethrography has routinely been termed sphincter spasm. This diagnosis was found to be incorrect in a significant percentage of boys with myelodysplasia. Thirty boys with myelodysplasia and dysfunction of the lower urinary tract, in whom voiding cystourethrography revealed narrowing in the area of the external sphincter, underwent urodynamic evaluation. Incoordination of the bladder and the sphincter (sphincter spasm) was found in only 16 (52%). In the other 14 (48%), there was either relaxation of the sphincter or absent electrical activity; atrophy and fibrosis of the striated muscle of the external sphincter (documented in six patients) probably resulted from chronic denervation. Thus, voiding cystourethrography only reveals narrowing of the urethra in the region of the external sphincter. However, when it is combined with urodynamic evaluation, the dynamics of the function of the sphincter can be defined and a rational approach to treatment planned.