OBJECTIVES: To assess the test-retest reliability of the 400-m usual-pace walk test (400-MWT), and to determine whether the 4-m walk test predicts inability to walk 400 m. DESIGN: Observational. SETTING: Community, 20-m tract course. PARTICIPANTS: Sixty study participants (aged ≥ 65) were enrolled from the community and met the following eligibility criteria: self-reported difficulty in two or more of four functional domains (mobility and exercise tolerance, upper extremity function, basic self-care, higher functional tasks of independent living) and a score of 18 or higher on the Mini-Mental State Examination. METHODS: The 400-MWT and 4-m walk test were each repeated within 7 days. RESULTS: The mean age ± standard deviation of the study population was 84.3 ± 6.3; 88.3% were women. Nineteen participants (31.7%) failed both 400-MWTs, and 41 successfully completed both tests (kappa = 1). Mean walking speed for the 4-m test was 0.87 ± 0.18 m/s for those who completed the 400-MWT and 0.53 ± 0.17 m/s for those who failed (P <.001). The Spearman correlation coefficient between 4-m and 400-m walking speeds was 0.93. The estimated area under the receiver operating characteristic curve between 4-m walking speed and the ability to perform the 400-MWT was 0.91. The 4-m gait speed averaged less than 0.6 m/s in 80% of subjects who failed the 400-MWT. CONCLUSION: The test-retest reliability for inability to complete the 400-MWT is high. Four-m walking speed is highly predictive of ability to perform the 400-MWT. These findings may prove useful to future clinical trials and observational studies that involve assessment of mobility limitations in older adults.