Startle modulation was induced by prestimulation in 43, 6-11 year old boys with attention-deficit hyperactivity disorder (ADHD), 13 of whom were or had been enuretic, 17 age-matched enuretic boys, and 42 age-matched normal boys, using 60-ms and 120-ms prestimulation intervals and a 4000-ms continuous tone. There was a significant multivariate effect of enuresis on startle amplitude modulation. This effect was attributed primarily to the reduction of amplitude inhibition following the 120-ms prestimulation interval regardless of whether or not enuresis was associated with ADHD. There was no effect of ADHD on startle modulation by prestimulation. The inhibition following the 120-ms prestimulation interval in the enuretic boys was reduced to the level of 5-year-old normal children, suggesting a maturational component of the deficient startle inhibition. The neurophysiologic dysfunction underlying the deficient startle inhibition in enuresis, but not ADHD, is discussed in terms of a possible dysfunction of mesopontine reticular mechanisms mediating preattentive processing of signals associated with spinal reflexes involved in urinary bladder control.