Objective: To assess baseline and modulated acoustic startle responses in adolescent girls with posttraumatic stress disorder (PTSD). Method: Twenty-eight adolescent girls with PTSD and 23 healthy control girls were recruited for participation in the study. Acoustic stimuli were bursts of white noise of 104 dB presented biaurally through headphones. Baseline startle responses as well as prepulse inhibition, a 1,000-Hz prestimulation tone presented 120 milliseconds before the startle stimulus for 30 milliseconds, and prepulse facilitation, a 1,000-Hz prestimulation tone presented continuously for 2,000 milliseconds before the startle stimulus, were compared in these two groups of girls. Results: At baseline and under neutral testing conditions, the magnitude of the startle response (eye blink) did not differ significantly between girls with PTSD and healthy control girls. There were no significant differences in the degree of prepulse inhibition or facilitation between the two groups of girls. Conclusions: Unlike combat veterans with PTSD, adolescent girls with PTSD who report exaggerated startle may not have exaggerated baseline acoustic startle responses in the laboratory. Further research should explore whether girls with PTSD demonstrate altered startle responses under stress and/or evidence of other types of psychophysiological abnormalities.