Background: In the WHO classification, small cell lung cancer (SCLC) and large cell neuroendocrine carcinoma (LCNEC) are considered as high-grade neuroendocrine carcinoma (HGNEC) of the lung. Although there were no randomized trials evaluating adjuvant chemotherapy for patients (pts) with resected HGNEC, EP was considered to be a standard regimen for this population. A phase III study showed the superiority of IP to EP in pts with extensive stage SCLC (JCOG9511). Methods: Pts with completely resected HGNEC were randomized in a 1:1 ratio to receive either etoposide (100 mg/m², days 1-3)/cisplatin (80 mg/m², day 1) or irinotecan (60 mg/m², days 1, 8, 15)/cisplatin (60 mg/m², day 1), using the minimization method according to sex, pathologic stage, histology and institution. The primary endpoint was changed from overall survival (OS) to relapse-free survival (RFS) during the study period. We assumed a 3-year RFS of 59% of EP arm and 72% of IP arm (hazard ratio (HR) of 0.623). Planned sample size was 220 in total to give a power of 80% with a one-sided alpha of 5%, an accrual period of 6 years and a follow-up period of 3 years. Results: Between April 2013 and October 2018, 221 pts with a median age of 66 years, pathological stage I (54%), SCLC (53%), were randomly assigned to the EP arm (n = 111) or the IP arm (n = 110). In the second interim analysis, the predictive probability that IP would be superior to EP at the time of the primary analysis was 15.9%, which led to early termination of the trial. With a median follow-up of 24.1 months, 3-year RFS was 65.4% versus 69.0% with HR of 1.076 (95% CI, 0.666-1.738; log-rank test, one-sided P= 0.619). In the subgroup analyses of histology, 3-year RFS in SCLC was 65.2% versus 66.5% with HR of 1.029 (95% CI, 0.544-1.944), and 3-year RFS in LCNEC was 66.5% versus 72.0% with HR of 1.072 (95% CI, 0.517-2.222). Overall survival at 3 years was 84.1% versus 79.0% with HR of 1.539 (95% CI, 0.760-3.117). Proportions of treatment completion were 87.4% (EP) and 72.7% (IP). Incidences (EP/IP) of grade 3 or 4 febrile neutropenia (20.2/3.7%) or neutropenia (97.2/35.8%) were more common in EP. Grade 3 or 4 diarrhea (0.9/8.3%) or anorexia (6.4/11.1%) were more common in IP. One treatment-related death due to tracheal bleeding was observed in IP. Conclusions: This study failed to show the superiority of IP to EP in RFS for pts with completely resected HGNEC. EP is still a standard treatment for this population. Clinical trial information: UMIN000010298.