To evaluate whether an activity monitor based on body acceleration measurement can accurately assess the energy cost of the human locomotion, 12 subjects walked a combination of three different speeds (preferred speed+/-1 km/h) and seven slopes (-15 to + 15% by steps of 5%) on a treadmill. Body accelerations were recorded using a triaxial accelerometer attached to the low back. The mean of the integral of the vector magnitude (norm) of the accelerations (mIAN) was calculated. (V) over dot O-2 was measured using continuous indirect calorimetry. When the results were separately analysed for each incline, mIAN was correlated to V. O2 (average r = 0.87, p < 0.001, n = 36). (V) over dot O-2 was not significantly correlated to mIAN when data were globally analysed (n = 252). Large relative errors occurred when predicted V. O2 (estimated from data of level walking) was compared with measured (V) over dot O-2 for different inclines (- 53% at +15% incline, to + 55% at - 15% incline). It is concluded that without an external measurement of the slope, the standard method of analysis of body accelerations cannot accurately predict the energy cost of uphill or downhill walking.