OBJECTIVE: High rates of medical morbidity have been reported in subjects with posttraumatic stress disorder (PTSD). The authors examined immune function in subjects in remission from past PTSD. METHOD: The initial study group was composed of 1,550 Japanese male workers. Japanese versions of the Events Check List, the Impact of Event Scale—Revised, and the Diagnostic Interview Schedule for DSM-IV were used to identify subjects who had a past history of PTSD. Twelve of the workers were identified as having such a history. These men were matched in age and smoking habits, which affect immunity, to 48 comparison subjects who had similar stressful life experiences but no current or past history of PTSD. Natural killer (NK) cell activity, lymphocyte subset counts, and production of interferon γ (IFN-γ) and interleukin-4 (IL-4) were measured in the 60 men by means of phytohemagglutinin stimulation. RESULTS: The number of lymphocytes, number of T cells, NK cell activity, and total amounts of IFN-γ and IL-4 were significantly lower in the 12 men with a past history of PTSD. CONCLUSIONS: PTSD leaves a long-lasting immunosuppression and has long-term implications for health.