OBJECTIVE: Psychological trauma leads to posttraumatic stress disorder (PTSD) in susceptible subjects. The aim of this study was to investigate the differences in regional cerebral blood flow (rCBF) between two groups of subjects exposed to different types of traumatic stressor either developing or not developing PTSD. METHODS: Twenty subjects developing (S) and 27 not developing (NS) PTSD after being exposed to either earlier person-under-the-train accident (NA) or being assaulted in the underground environment (A) were included in the study. 99mTc-HMPAO SPECT was performed and the uptake in 29 regions of the brain (VOIs), bilaterally, was assessed. rCBF distribution was compared, using analysis of variance (ANOVA), between groups (S/NS) and type (A/NA) during a situation involving an auditory evoked re-experiencing of the traumatic event. Discriminant analysis was applied to test the concordance between clinical diagnosis and SPECT findings. RESULTS: In the general analyses significant differences were found between groups and types and there was a significant hemisphere x type interaction. S showed higher CBF than NS and so did A as compared to NA, particularly in the right hemisphere. Discriminant analysis correctly classified 66% of cases (p < 0001) in testing S/NS and 72% (p < 0001) in testing NA/A. CONCLUSIONS: Under recall of their traumatic experience we found higher relative CBF distribution values in S as compared to NS. CBF was higher in the right hemisphere and particularly in assaulted subjects. These findings underscore the role upon trauma recall of both the right hemisphere and the nature of the stressing event.