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Abstract Background/Aim For patients treated with osimertinib as first-line therapy, there have been no studies comparing both progression-free survival (PFS) and overall survival (OS) according to performance status (PS). Furthermore, no studies have examined differences in baseline genetic abnormalities between patients with poor and good PS. Therefore, we aimed to investigate differences in baseline genetic abnormalities and treatment effects between patients with poor and good PS who received osimertinib as the primary treatment. Patients and methods This is a secondary analysis of the ELUCIDATOR study, which is a multi-center prospective observational study in Japan that assessed mechanisms underlying resistance to osimertinib as first-line treatment for advanced non-small cell lung cancer with epidermal growth factor receptor mutations. Results There were 153 and 25 patients in the good and poor PS groups, respectively. Multivariate analysis revealed no significant between-group differences in PFS (hazards ratio [HR]: 0.98, 95% confidence interval [CI]: 0.52–1.72, p = 0.946). Multivariate analysis of OS revealed that poor PS was a poor prognostic factor (HR: 2.67, 95% CI: 1.43–4.73, p = 0.003). Regarding baseline genetic abnormalities, there was a significant increase in APC-positive cases (20.0% vs. 2.2%, p = 0.009) and a trend toward more CTNNB1-positive cases in the poor PS group than in the good PS group (14.3% vs. 2.9%, p = 0.062). Conclusion There was no between-group difference in PFS, although OS was significantly inferior in the poor PS group. Additionally, there was a significant increase in APC-positive cases and a trend toward more CTNNB1-positive cases in the poor PS group.