BACKGROUND: Osimertinib is the standard of care in the initial treatment for advanced epidermal growth factor receptor (EGFR) mutation-positive lung cancer. However, clinical data and reliable prognostic biomarkers are insufficient. METHODS: We performed a retrospective multicentre cohort study for 538 EGFR mutation-positive patients, who received osimertinib as the initial treatment between August 2018 and December 2019. The main outcome was progression-free survival (PFS). RESULTS: The median observation period was 14.7 months (interquartile range 11.4-20.0). The median PFS was 20.5 months (95% confidence interval [CI] 18.6-not reached). Multivariate analysis showed that sex (male) (hazard ratio [HR] 1.99, 95% CI 1.35-2.93, P = 0.001), malignant effusions (HR 1.51, 95% CI 1.11-2.04, P = 0.008), liver metastasis (HR 1.55, 95% CI 1.03-2.33, P = 0.037), advanced unresectable cases (HR 1.71, 95% CI, 1.04-2.82, P = 0.036), mutation type and programmed cell death-ligand 1 (PD-L1) expression were associated with PFS. The L858R (HR 1.55, 95% CI 1.01-2.38, P = 0.043) and uncommon mutations (HR 3.15, 95% CI 1.70-5.83, P < 0.001) were associated with PFS. PD-L1 expression of 1-49% (HR 1.66, 95% CI 1.05-2.63, P = 0.029), ≥50% (HR 2.24, 95% CI 1.17-4.30, P = 0.015) and unknown (HR 1.53, 95% CI 1.05-2.22, P = 0.026) was associated with PFS. The main reasons for treatment discontinuation among 219 patients were disease progression (44.3%), pneumonitis (25.5%) and other adverse events (16.0%). CONCLUSION: During initial treatment with osimertinib, PD-L1 expression is significantly related to PFS. Adverse events are a noteworthy reason for discontinuation.