Based on the hypothesis that pain is a stand-alone problem, not just a symptom of Post-Traumatic Stress Disorder (PTSD), the effect of group psycho-education (“pain school”) for survivors of the Khmer Rouge regime with pain-PTSD comorbidity was tested in Cambodia in 2015. After baseline assessment comprising pain-related measures (Brief Pain Inventory, Disability Rating Index) and measures for PTSD, anxiety, depression, and distress, 113 subjects were randomized to a waitlist control group (CG, n = 58) and a treatment group (TG, n = 55). After treatment TG improved significantly, with clinically relevant effect size. Effect size was, however, substantially lower than in two prior pilot trials, and the improvement was not maintained at six-month follow-up. The main reason for this is hypothesized to be that the intervention had been delivered in too condensed a format. It is concluded that treatment addressing pain can also ameliorate mental health problems, implying that more attention should be paid to pain treatment for subjects suffering from pain/PTSD comorbidity.