Abstract only 8500 Background: Adjuvant platinum-based chemotherapy (chemo) provides only a modest 5-year survival benefit in fully resected, high-risk early-stage NSCLC. We report the primary disease-free survival (DFS) results from the pre-planned interim analysis of IMpower010, a randomized phase 3 open-label trial of adjuvant atezolizumab (atezo; anti–PD-L1) vs best supportive care (BSC) after adjuvant chemo in patients (pts) with early-stage resected NSCLC. Methods: Eligible pts had completely resected (4-12 weeks prior to enrollment) Stage IB (≥4 cm)-IIIA NSCLC (AJCC/UICC v7) and ECOG PS 0-1. A total of 1280 pts were enrolled, and 1269 pts received up to four 21-day cycles of cisplatin-based chemo (plus pemetrexed, docetaxel, gemcitabine or vinorelbine). Of these pts (n=1269), 1005 were subsequently randomized 1:1 to 16 cycles of atezo 1200 mg Q3W or BSC. The primary endpoint of investigator-assessed DFS and secondary endpoint of overall survival (OS) were tested hierarchically: first DFS in the PD-L1 TC ≥1% (SP263) subgroup with Stage II-IIIA disease, then DFS in all randomized pts with Stage II-IIIA disease, then DFS in the ITT population (Stage IB-IIIA) and finally OS in the ITT population. Efficacy assessments were based on randomized pts. Safety was assessed in the safety-evaluable population, defined as pts who received ≥1 dose of atezo or who had ≥1 post-baseline safety assessment if randomized to the BSC arm. Results: At data cutoff (January 21, 2021), median follow-up was 32.2 months in the ITT population. Baseline characteristics were generally balanced between arms. Atezo showed statistically significant DFS benefit vs BSC in the PD-L1 TC ≥1% Stage II-IIIA and all randomized Stage II-IIIA populations; the significance boundary was not crossed for DFS in the ITT population (Table). OS data were immature and not formally tested. Pts in the atezo arm received a median of 16 (range, 1-16) atezo doses. Any-grade AEs occurred in 92.7% (atezo) and 70.7% (BSC); events were Grade 3/4 in 21.8% and 11.5%, respectively. Grade 5 treatment-related AEs occurred in 0.8% of pts in the atezo arm. AEs leading to atezo discontinuation occurred in 18.2% of atezo-treated pts. Conclusions: IMpower010 met its primary endpoint, showing DFS benefit with adjuvant atezo vs BSC after adjuvant chemo in pts with resected Stage II-IIIA NSCLC, with pronounced benefit in the PD-L1 TC ≥1% subgroup. The safety profile of atezo was consistent with prior experience of atezo monotherapy across indications and lines of therapy. Funding: F. Hoffmann-La Roche Ltd. Clinical trial information: NCT02486718. Table: see text