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 diVerent 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. [Vdot]O2 was measured using continuous indirect calorimetry. When the results were separately analysed for each incline, mIAN was correlated to [Vdot]O2 (average r = 0.87, p < 0.001, n = 36). [Vdot]O2 was not significantly correlated to mIAN when data were globally analysed (n = 252). Large relative errors occurred when predicted [Vdot]O2 (estimated from data of level walking) was compared with measured [Vdot]O2 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.