PURPOSE: Trimodality therapy, comprised of induction chemoradiotherapy (iCRT) followed by surgery, is a highly invasive treatment option for locally advanced non-small cell lung cancers (LA-NSCLCs; defined as a heterogenous disease). We conducted this study to investigate the prognostic nutritional index (PNI) of LA-NSCLC patients undergoing trimodality therapy, which has not been studied in detail before. METHODS: The subjects of this retrospective study were 127 patients who underwent trimodality therapy between 1999 and 2016. We measured the PNI at three points: before iCRT (pre-iCRT), before the operation, and after the operation. RESULTS: PNIs decreased significantly as treatment progressed. Patients with clinical T3/4 (cT3/4) disease had a significantly lower PNI than those with cT1/2 disease, but the extent of lymph-node metastasis did not affect the PNI at any point. Using the cut-off values of receiver-operating curve analyses, multivariable analyses revealed that a high PNI pre-iCRT correlated significantly with a better survival of LA-NSCLC patients, especially those with cT3/4 disease (hazard ratio 3.84; 95% confidential interval 1.34-12.5, P = 0.012). CONCLUSIONS: Measuring the PNI before trimodality therapy is important for predicting the clinical outcome of patients with LA-NSCLC, with differing predictive ability according to the disease extent. Perioperative intensive nutritional intervention must be considered for patients who undergo trimodality therapy for LA-NSCLC.