Exaggerated startle is reputed to be one of the cardinal symptoms of posttraumatic stress disorder (PTSD); however, objective studies have given conflicting results as to whether or not startle is increased in PTSD. The present study investigated startle in PTSD during the threat of shock (fear-potentiated startle). The eyeblink component of the startle reflex was measured at various times preceding and following the anticipation of unpleasant electric shocks in 9 PTSD subjects and 10 age-matched, healthy controls. Startle amplitude was significantly greater during baseline and during shock anticipation in the PTSD subjects, compared to the controls. Habituation of the startle reflex was normal. Because other studies in the literature, as well as in our own laboratory, have failed to find exaggerated startle at baseline (i.e., absence of stress) in PTSD patients, it is unlikely that the present results reflect a chronic elevation of startle in this group. Instead, the higher levels of startle in the PTSD group probably resulted from a greater conditioned emotional response in this group, triggered by anticipation of electric shocks that generalized to the unfamiliar experimental context in which testing occurred. Hence, emotionally charged test procedures may be especially informative in distinguishing PTSD patients from other psychiatric diagnostic groups.