Epidemiologic studies have reported that the majority of community residents in the United States have experienced posttraumatic stress disorder (PTSD)-level traumatic events, as defined in the DSM-IV. Only a small subset of trauma victims develops PTSD (<10%). Increased incidence of other disorders following trauma exposure occurs primarily among trauma victims with PTSD. Female victims of traumatic events are at higher risk for PTSD than male victims are. Direct evidence on the causes of the sex difference in the conditional risk of PTSD is unavailable. The available evidence suggests that the sex difference is not due to (a) the higher occurrence of sexual assault among females, (b) prior traumatic experiences, (c) preexisting depression or anxiety disorder, or (d) sex-related bias in reporting. Observed sex differences in anxiety, neuroticism, and depression, inducing effects of stressful experiences, might provide a theoretical context for further inquiry into the greater vulnerability of females to PTSD.