The purpose was to assess the validity of a novel Activity Monitor to quantify physical activity in congestive heart failure. The Activity Monitor is based on long-term ambulatory monitoring of signals from body-fixed accelerometers. Information can be obtained on which mobility-related activity is performed, when, how intense, and for how long. Ten patients performed several functional activities. Continuous registrations of accelerometer signals were made and the output was compared with visual analysis of simultaneously made video recordings (reference method). Overall results showed an agreement between both methods of 90%. Percentages of sensitivity and predictive value were higher than 80% for most activities. Overall number of transitions was determined well (Activity Monitor, 153; video, 149; p = 0.33). It was concluded that the Activity Monitor is a valid instrument to quantify several aspects of everyday physical activity in congestive heart failure.