In the PACIFIC trial, durvalumab significantly improved progression-free and overall survival (PFS/OS) versus placebo, with manageable safety, in unresectable, stage III non-small-cell lung cancer (NSCLC) patients without progression after chemoradiotherapy (CRT). We report exploratory analyses of outcomes by tumour cell (TC) programmed death-ligand 1 (PD-L1) expression. Patients were randomly assigned (2:1) to intravenous durvalumab 10 mg/kg every 2 weeks or placebo ≤12 months, stratified by age, sex, and smoking history, but not PD-L1 status. Where available, pre-CRT samples were tested for PD-L1 expression (immunohistochemistry) and scored at pre-specified (25%) and post hoc (1%) TC cut-offs. Treatment-effect hazard ratios (HRs) were estimated from unstratified Cox proportional hazards models (Kaplan–Meier-estimated medians). In total, 713 patients were randomly assigned, 709 of whom received at least 1 dose of study treatment durvalumab (n = 473) or placebo (n = 236). Some 451 (63%) were PD-L1-assessable: 35%, 65%, 67%, 33%, and 32% had TC ≥25%, <25%, ≥1%, <1%, and 1%–24%, respectively. As of 31 January 2019, median follow-up was 33.3 months. Durvalumab improved PFS versus placebo (primary-analysis data cut-off, 13 February 2017) across all subgroups HR, 95% confidence interval (CI); medians: TC ≥25% (0.41, 0.26–0.65; 17.8 versus 3.7 months), <25% (0.59, 0.43–0.82; 16.9 versus 6.9 months), ≥1% (0.46, 0.33–0.64; 17.8 versus 5.6 months), <1% (0.73, 0.48–1.11; 10.7 versus 5.6 months), 1%–24% 0.49, 0.30–0.80; not reached (NR) versus 9.0 months, and unknown (0.59, 0.42–0.83; 14.0 versus 6.4 months). Durvalumab improved OS across most subgroups (31 January 2019 data cut-off; HR, 95% CI; medians): TC ≥ 25% (0.50, 0.30–0.83; NR versus 21.1 months), <25% (0.89, 0.63–1.25; 39.7 versus 37.4 months), ≥1% (0.59, 0.41–0.83; NR versus 29.6 months), 1%–24% (0.67, 0.41–1.10; 43.3 versus 30.5 months), and unknown (0.60, 0.43–0.84; 44.2 versus 23.5 months), but not <1% (1.14, 0.71–1.84; 33.1 versus 45.6 months). Safety was similar across subgroups. PFS benefit with durvalumab was observed across all subgroups, and OS benefit across all but TC <1%, for which limitations and wide HR CI preclude robust conclusions. •Tumour tissue acquisition (pre-chemoradiotherapy) and tumour cell (TC) PD-L1 expression testing were not mandated.•However, outcomes were assessed based on PD-L1 expression in subgroups defined by pre-specified and post hoc TC cut-offs.•Treatment benefit with durvalumab, versus placebo, was observed irrespective of PD-L1 expression in terms of PFS.•OS improvement was demonstrated overall and across all subgroups, apart from the post hoc TC <1% subgroup.•Limitations (few events and baseline imbalances) and a wide CI for OS HR (includes 1) prevent robust conclusions for TC <1%.