<i>Background:</i> The DynaPort Activity Monitor (DAM) has been reported to be useful to evaluate the activity in healthy subjects and patients with chronic obstructive pulmonary disease (COPD). However, it is difficult to estimate the activity of COPD patients using DAM, because its battery works only for several hours and sensors should be worn at two parts of the body. A newly developed compact, single-position triaxial accelerometer (Actimarker) can measure the activity for >1 month, but has not been validated for COPD patients. <i>Objectives:</i> The validity of the Actimarker was evaluated in COPD patients. <i>Methods:</i> In study 1, the validity of the device was tested in 14 stable COPD patients by comparing it with DAM. In study 2, the influence of the weather on activity was examined. In study 3, the number of measurement days required to ensure repeatability was determined. <i>Results:</i> The durations of activity measured by the Actimarker and DAM were significantly correlated at intensity values ≧2.0, ≧2.5 and ≧3.0 METs. The duration of activity on rainy days was significantly shorter than that on non-rainy days. The values of intraclass correlation coefficients were >0.8 in 3-, 4- or 5-day measurements, and there was no systematic bias at any number of days or intensities with Bland-Altman plots. <i>Conclusions:</i> The validity of the Actimarker was confirmed, and repeatability was obtained when the data from at least 3 non-rainy weekdays were analyzed. Actimarker appears to be useful as a simplified method to evaluate the physical activity of COPD patients.