OBJECTIVE:The aim of this study was to determine the duration of physical activity (PA) monitoring required for reliable measurements following stroke. DESIGN:Single-center, prospective, observational study. SETTING:PA was measured in a community setting. PARTICIPANTS:Adults (N=70) poststroke. MAIN OUTCOME MEASURES:The SenseWear armband was used to monitor PA for 5 days (≥10 hours wear per day). DATA ANALYSIS:Variance among 2, 3, 4, and 5 days of consecutive measurements for PA variables was examined using intraclass correlation coefficients (ICCs). The minimum number of days to achieve acceptable reliability (ICC ≥0.8) was calculated. Differences between weekdays and weekend days were investigated using paired t tests and Wilcoxon signed rank tests. RESULTS:Two days of measurement was sufficient to achieve an ICC ≥0.8 for daily averages of total energy expenditure, step count, and time spent sedentary (≤1.5 metabolic equivalent tasks [METs]) and in light (1.5-3 METs) and moderate- to vigorous-intensity (>3 METs) PA. At least 3 days were required to achieve an ICC ≥0.8 when investigating the number of and time spent in bouts (≥10 minutes) of moderate to vigorous PA and sedentary behavior. Participants took significantly more steps (P=.03) and spent more time in light PA (P=.03) on weekdays than weekends. CONCLUSION:Following stroke, 2 days of measurement appears sufficient to represent habitual PA for many simple variables. Three or more days may be necessary for reliable estimates of bouts of PA and sedentary behavior. Consistent inclusion or exclusion of a weekend day is recommended for measuring step count and light PA. Short periods of monitoring provide reliable PA information and may make PA measurement more feasible in the clinical setting.