OBJECTIVE: To analyze the reliability, validity, and determinants of the incremental shuttle walk test (ISWT) in adults with noncystic fibrosis bronchiectasis. DESIGN: Cross-sectional study. SETTING: Outpatient clinic. PARTICIPANTS: Subjects (N=75; 26 men) underwent, on different days, cardiopulmonary exercise testing (CPET) and 2 ISWTs, 30 minutes apart. The number of steps in daily life was recorded. Concurrent validity was tested by the relation between distance walked with peak load and oxygen consumption (V˙o2). INTERVENTIONS: None. MAIN OUTCOME MEASURES: Distance walked (m) was compared between the first and second ISWTs; greatest distance walked was correlated with peak load and Vo2peak obtained from CPET, steps per day, and dyspnea evaluated by the Medical Research Council (MRC) scale; and desaturation was compared between CPET and the ISWT. RESULTS: Distance walked was equivalent between the first ISWT (441±152m) and the second ISWT (445±153m) with an excellent intraclass correlation coefficient (.995; 95% confidence interval, .99-.997). There were significant correlations between distance walked and peak load (r=.82), V˙o2 (r=.72), steps per day (r=.61), and the MRC scale (r=-.69). Age, body mass index, sex, forced vital capacity (% predicted), dyspnea, and steps per day explained 70% of the variation in distance walked (m) and 60% of the variance when expressed as percent predicted. Higher desaturation was observed during the ISWT (-4%±4%) than cycling (-2±3%) (P<.001). CONCLUSIONS: The ISWT is reliable, represents functional capacity, and induces greater desaturation than cycling. Age, body composition, pulmonary function, dyspnea, and physical activity in daily life are determinants of the distance walked on the ISWT.