Background Clinicians worldwide use the Eastern Cooperative Oncology Group Performance Score (PS) to evaluate the patient’s suitability to the appropriate anti-neoplastic treatment. The scale is outdated and can be in uenced by the doctor’s professional experience, interactions between patient and doctor, and the patient’s socioeconomic position. An over- or underestimation of PS has potentially fatal outcomes for the patients who either may not receive life prolonging treatment due to underestimation or experience unreasonable and potentially fatal complications due to overestimation. To our knowledge, no areas of specializations, other than oncology, are basing comprehensive medical decisions on subjective measures. Aim To investigate the feasibility of a 6MWT to predict complications to rst-line treatment. Material and methods Patients with small cell lung cancer (limited disease and extensive disease), or non- small lung cancer (NSCLC) stage I-IV were recruited. Patients were excluded if they had received anti-neoplastic treatment within ve years, had other cancer diagnoses (including mesothelioma), or if they were not ambulatory. The patients were tested with the 6-minute walk test (6MWT) close to the time of the diagnosis. Complications to rst-line treatment were collected with standardized guidelines and analyzed. Results The acceptance rate was 46.7 % (21/45 patients included). All patients were diagnosed with NSCLC (stages I and II: n=5, stage III: n=9, and stage IV: n=4). Response to rst-line treatment were categorized into no, minor and major complications. The patients with no complications walked 530 m ± 68 m, those with minor complications walked 436 m ± 61 m. and those with major complications walked 360 m ± 136 m. (p=0.043). PS did not re ect similar association (p=0.312), and several of the patients experiencing minor and major complications to the treatment were estimated in PS0. Conclusion The results from the present feasibility study shows that the 6MWT is feasible in newly diagnosed patients with lung cancer and shows statistical trends toward the 6MWTs ability to predict complications to rst-line treatment that should be investigated in a larger homogenous trial.