Children born with myelomeningocele face the morbidity of urinary tract complications after they have survived the neurologic complications of the first years of life. The incidence of urinary tract complications was evaluated in 258 children before any operation or intermittent catheter management was performed. In 119 children under one year of age the incidence of urinary tract infection of more than 105 colonies was 23 per cent, reflux was 22 per cent, and upper tract dilatation was 6 per cent. In children of all ages the incidence of urinary tract infection of more than 105 colonies was 26 per cent, reflux 34 per cent, upper tract dilatation 18 per cent, and urinary incontinence 87 per cent. In 11 children with complete denervation of the pelvic floor muscles ref lux developed in 2 (18 %) and upper tract dilatation in 1 (9 %). In 13 children with slight or moderate denervation of the pelvic floor muscles reflux developed in 5 (38%) and upper tract dilatation in 7 (54 %). The development of upper tract changes with myelomeningocele was mainly related to some innervation of the pelvic floor muscles (including periurethral striated muscle) with incoordinate detrusor/sphincter activity. These changes mainly occurred in the first two to four years of life.