PURPOSE: This study compared the mental symptoms, especially symptoms of posttraumatic stress disorder (PTSD), of women who escaped prostitution, helping activists at shelters, and matched control subjects. MATERIALS AND METHODS: We assessed 113 female ex-prostitutes who had been living at a shelter, 81 helping activists, and 65 control subjects using self-reporting questionnaires on demographic data, symptoms related to trauma and PTSD, stress-related reactions, and other mental health factors. RESULTS: Female ex-prostitutes had significantly higher stress response, somatization, depression, fatigue, frustration, sleep, smoking and alcohol problems, and more frequent and serious PTSD symptoms than the other 2 groups. Helping activists also had significantly higher tension, sleep and smoking problems, and more frequent and serious PTSD symptoms than control subjects. CONCLUSION: These findings show that engagement in prostitution may increase the risks of exposure to violence, which may psychologically traumatize not only the prostitutes themselves but also the people who help them, and that the effects of the trauma last for a long time. Future research is needed to develop a method to assess specific factors that may contribute to vicarious trauma of prostitution, and protect field workers of prostitute victims from vicarious trauma.