Symptom exacerbation (i.e., treatment side effects) has often been neglected in the psychotherapy literature. Although prolonged exposure has gained empirical support for the treatment of chronic posttraumatic stress disorder (PTSD), some have expressed concem that imaginal exposure, a component of this therapy, may cause symptom exacerbation, leading to inferior outcome or dropout. In the present study, symptom exacerbation was examined in 76 women with chronic PTSD. To define a "reliable" exacerbation, we used a method incorporating the standard deviation and test-retest reliability of each outcome measure. Only a minority of participants exhibited reliable symptom exacerbation. Individuals who reported symptom exacerbation benefited comparably from treatment. Further, symptom exacerbation was unrelated to dropout. Thus, although a minority of individuals experienced a temporary symptom exacerbation, this exacerbation was unrelated to outcome.