Psychological distress is common in lung cancer patients with a poor prognosis. The present study aims to investigate the efficacy of collaborative care for patients with newly diagnosed inoperable lung cancer in South Korea. The study is a three-arm parallel-groups nonrandomized clinical trial with an active arm that includes distressed patients who receive collaborative care, one comparison arm that includes distressed patients who receive enhanced usual care, and another comparison arm that includes non-distressed patients. In total, 267 consecutive patients newly diagnosed with medically inoperative lung cancer will be recruited. The primary outcomes are the changes in Hospital Anxiety and Depression Scale-depression and the Distress Thermometer at 12 and 32 weeks after enrollment. Sub-analyses of patients in the active arm of the study will include a comparison of the efficacy of a combination of oral antidepressant (escitalopram) treatment and collaborative care versus that of collaborative care alone.