Objective: Respiratory illness and posttraumatic stress disorder (PTSD) are common debilitating conditions that frequently co-occur. Observational studies indicate that PTSD, independently of smoking, is a major risk and maintenance factor for lower respiratory symptoms (LRS). The current study experimentally tested this etiologic pathway by investigating whether LRS can be reduced by treating PTSD symptoms. Method: Ninety daily smokers exposed to the World Trade Center (WTC) disaster (mean age 50 years, 28% female; 68% White) completed 8-session group-based weekly comprehensive trauma management and smoking cessation treatment that focused on skills to alleviate PTSD symptoms. LRS, PTSD symptoms, and smoking were assessed weekly. Data was analyzed using multilevel models of within-person associations between LRS, PTSD symptoms, smoking, and treatment dose across 8 weekly sessions with concurrent and lagged outcomes. Results: LRS improved significantly with treatment (reduction of .50 standard deviations). Reduction in PTSD symptoms uniquely predicted improvement in LRS at consecutive sessions 1 week apart and fully accounted for the treatment effect on LRS. The effect of PTSD symptoms was stronger than that of smoking, and the only effect to remain significant when both entered the model. Notably, reduction in LRS did not predict future improvement in PTSD symptoms. Conclusions: The results are in line with the etiologic pathway suggesting that PTSD symptoms are a risk and maintenance factor for chronic LRS and that treatment of PTSD can help to alleviate LRS in trauma-exposed populations. PTSD is emerging as a novel and important treatment target for chronic respiratory problems.