Startle responses to bursts of white noise were recorded as blink reflexes 17-21 months after a traumatic event in six children with posttraumatic stress disorder (PTSD) and in six normal control children. A seventh child with PTSD was studied on four occasions during the 2 years following a stressful event. The startle responses were modulated by nonstartling acoustic prestimulation in order to study the inhibitory and facilitatory modulation of startle reaction by brainstem mechanisms. The children with PTSD experienced a significant loss of the normal inhibitory modulation of startle response, suggesting that the traumatic experience had induced a long-lasting brainstem dysfunction.