Usual gait speed is a consistent predictor of adverse outcomes in community-dwelling elderly people. The reliability and validity of the 4-m gait speed (4MGS) has recently been demonstrated in patients with chronic obstructive pulmonary disease (COPD). The aims of this study were to assess the responsiveness of the 4MGS and to estimate the minimal clinically important difference (MCID). In 301 COPD patients, 4MGS and incremental shuttle walk (ISW) were measured before and after pulmonary rehabilitation. 4MGS and ISW were also measured at baseline and 1 year later in a separate cohort of 162 COPD patients. The MCID of 4MGS was estimated using distribution and anchor-based methods. 4MGS improved significantly with pulmonary rehabilitation (mean change 0.08 m·s(-1), p<0.001). The minimal detectable change at 95% confidence was 0.11 m·s(-1). The MCID was estimated at 0.11 m·s(-1) (anchored against ISW) and 0.08 m·s(-1) (anchored against self-reported improvement). The effect size for 4MGS was greatest in frail individuals. After 12 months, mean 4MGS declined by 0.04 m·s(-1). When anchored against a decline of more than the MCID for ISW, change in 4MGS was -0.11 m·s(-1). The 4MGS is responsive to pulmonary rehabilitation and longitudinal change in COPD, and has potential as a simple functional assessment tool in COPD. The 4MGS may be particularly useful in frail individuals with COPD.