Although an exaggerated startle response is a symptom of posttraumatic stress disorder (PTSD), empirical support for elevated baseline startle in PTSD has been weak. The present study investigated the eyeblink component of the acoustic startle reflex and prepulse inhibition (PPI) in 21 unmedicated Vietnam veterans with PTSD and in 17 civilian and 10 combat veteran comparison subjects. Patients with PTSD exhibited normal acoustic startle amplitude, but showed a significant reduction in PPI relative to the civilian subjects. There was only a trend toward a reduction in PPI in the PTSD group compared with the combat control group. The study does not support the hypothesis of exaggerated baseline startle in Vietnam veterans with PTSD but suggests abnormal startle modulation by a prepulse (i.e., PPI). Discrepancies between studies concerning the amplitude of startle in PTSD are discussed.