BACKGROUND: Current guidelines do not routinely recommend adjuvant therapy for resected stage I large cell lung neuroendocrine cancer (LCNEC). However, data regarding the role of adjuvant therapy in early LCNEC are limited. This National Cancer Database (NCDB) analysis was performed to improve the evidence guiding adjuvant therapy for early LCNEC. METHODS: Overall survival (OS) of patients with pathologic T1-2a N0 M0 LCNEC who underwent resection in the NCDB from 2003 to 2015 was evaluated with Kaplan-Meier and multivariable Cox proportional hazards analyses. Patients who died within 30 days of surgery and with more than R0 resection were excluded. RESULTS: Of 2642 patients meeting study criteria, 481 (18%) received adjuvant therapy. Adjuvant chemotherapy in stage IB patients was associated with a significant increase in OS (hazard ratio, 0.67; 95% confidence interval, 0.50 to 0.90). However, there was no significant difference in survival between adjuvant chemotherapy and no adjuvant therapy for stage IA LCNEC (hazard ratio, 0.92; 95% confidence interval, 0.75 to 1.11). Adjuvant radiotherapy, whether alone or combined with chemotherapy, was not associated with a change in OS. In subgroup analysis, patients receiving adjuvant chemotherapy after lobar resection for stage IB LCNEC had a significant survival benefit compared with patients not receiving adjuvant therapy. CONCLUSIONS: In early-stage LCNEC, adjuvant chemotherapy appears to confer an additional overall survival advantage only in patients with completely resected stage IB LCNEC and not for patients with completely resected stage IA LCNEC.