Posttraumatic stress disorder (PTSD) patients with histories of cocaine and alcohol abuse (CA-PTSD) were compared with normal volunteers. Positron emission tomography (PET) scans with 15O-butanol were used to compare regional cerebral blood flow (rCBF) between the groups during rest and during an auditory continuous performance task (ACPT). CA-PTSD patients had significantly higher rCBF in right amygdala and left parahippocampal gyrus than normals during the ACPT. Normals had higher rCBF at frontal cortex during the resting scan and during the ACPT. The role of the amygdala in attention and fear conditioning suggests that increased amygdala rCBF may be related to clinical features of PTSD. Cocaine use may be associated with increased amygdala rCBF in PTSD patients. Amygdala and frontal cortex attention system components may be reciprocally related and their relative contributions to processing of neutral stimuli perturbed in CA-PTSD.