BACKGROUND : Physical activity measures are valuable for assessing the progression of chronic respiratory diseases. The 4-m gait speed (4MGS) test is an established functional assessment in the elderly. However, the relationship between the 4MGS and daily activity in patients with chronic respiratory diseases has not been fully understood. The present study aimed to investigate whether the 4MGS predicted daily activity, including physical activity level (PAL), in patients with chronic respiratory diseases. METHODS : We enrolled 57 patients with chronic respiratory diseases, including interstitial lung disease and chronic obstructive pulmonary disease, and evaluated the correlations between the 4MGS and various clinical parameters, including respiratory function, the 6-min walk test (6MWT), and daily activities, by using an accelerometer. Linear regression analysis was performed to identify significant predictors of daily activity. RESULTS : The 4MGS was significantly correlated with daily step counts and PAL, as well as the 6 min walk distance (r = 0.477, p < 0.001; r = 0.433, p = 0.001; and r = 0.593, p < 0.001, respectively). In the multivariate linear regression analysis, the 4MGS, % predicted forced expiratory volume in 1 s, and body mass index were independent predictors of PAL. Receiver operating characteristic analysis revealed that a 4MGS <1.07 m/s was the optimal cutoff for predicting an inactive PAL (area under the curve, 0.728; 95% confidence interval, 0.589-0.866). Patients with a slower 4MGS had significantly reduced daily activity than did those with a preserved 4MGS, despite similar modified Medical Research Council dyspnea scale measures and respiratory parameters, such as oxygenation profiles. CONCLUSIONS : The 4MGS test is a simple screening test and a useful predictor of worsening daily activity in patients with chronic respiratory diseases.