The 6-minute walk distance (6MWD) is one of the most commonly used measures of functional capacity in lung cancer, however, the minimal important difference (MID) has not been established. The aims of this exploratory study are, in lung cancer, to estimate (1) the MID of the 6MWD and (2) relationship between 6MWD, demographic and disease-related factors. Fifty-six participants with stage I–IV lung cancer completed the 6MWD prior to treatment and 10 weeks later. No exercise intervention occurred. Additional measures included European Organization for Research and Treatment of Cancer questionnaire (EORTC-QLQ-C30) and questionnaires assessing function, physical activity and symptoms. MID was calculated using anchor- and distribution-based methods. The mean 6MWD decline in participants classed as deteriorated was 60 m compared with 16 m in participants classed as not-deteriorated ( p = 0.01). The receiver operating curve indicated a cut-off value for clinically relevant change to be 42 m (95% confidence interval (CI) 6–75) (area under curve = 0.66, 95% CI 0.51–0.81) or a 9.5% change. Distribution-based methods indicated an MID between 22 m (95% CI 18–26) and 32 m (95% CI 20–42). Higher 6MWD correlated with better function ( r = −0.42, p = 0.001), physical activity ( r = 0.56, p < 0.005) and dyspnoea ( r = −0.44, p = 0.001). The MID for deterioration of the 6MWD in lung cancer is estimated to be between 22 m and 42 m or a change of 9.5%.