Subjects on an addiction treatment unit who had been exposed to severe combat conditions in Vietnam were screened for posttraumatic stress disorder (PTSD). Of 24 with PTSD, 58.3% carried the D2A1 allele. Of the remaining eight who did not meet PTSD criteria, 12.5% carried the D2A1 allele (p = 0.04). In a replication study of 13 with PTSD, 61.5% carried the D2A1 allele. Of the remaining 11 who did not meet criteria for PTSD, 0% carried the D2A1 allele (p = 0.002). For the combined group 59.5% of those with PTSD carried the D2A1 allele versus 5.3% of those who did not have PTSD (p = 0.0001). These results suggest that a DRD2 variant in linkage disequilibrium with the D2A1 allele confers an increased risk to PTSD, and the absence of the variant confers a relative resistance to PTSD.