Purpose Recently, based on results of the PACIFIC trial, durvalumab after chemoradiotherapy (CRT) became the standard therapy for unresectable stage III non-small cell lung cancer (NSCLC). However, in the PACIFIC trial, patients were recruited and randomized after CRT, and certain patients were considered ineligible after CRT in the real world. No study has been conducted on the patients who were ineligible for the PACIFIC trial, and hence, we conducted a retrospective study on them. Methods We identified 82 patients with stage III NSCLC who received definitive platinum-based concurrent CRT and had World Health Organization performance status of 0–1. We investigated the proportion, clinical characteristics, and prognoses of patients who became ineligible for the PACIFIC trial after CRT. Results After CRT, 19 of 82 patients (23%) became ineligible for the PACIFIC trial. Comparison between eligible and ineligible patients revealed that old age ( p  = 0.042), male gender ( p  = 0.031), and radiation therapy with V20 ≥ 35% ( p  = 0.032) were associated with ineligibility after CRT. Moreover, ineligible patients showed shorter PFS (6.6 vs. 15.7 months, hazard ratio HR 2.61, 95% confidence interval CI 1.16–5.89, p  = 0.016) and shorter OS (18.6 vs. 44.3 months, HR 3.03, 95% CI 1.29–7.10, p  = 0.007) than eligible patients. Conclusions Our study revealed the clinical characteristics and prognoses of patients who became ineligible for the PACIFIC trial after CRT. Physicians should be careful while prescribing CRT for patients with characteristics such as old age, male gender, and radiation therapy with V20 ≥ 35%.