Ground reaction forces from two force plates are used to calculate the cyclic oscillations of the body centre of mass of subjects walking at preferred speed. Good approximations to the oscillations may be obtained from formulae containing just the first- and second-order Fourier coefficients of the ground reaction forces taken over a complete walking cycle. The symmetric components of the oscillations have consistent mutual phase relations for normal subjects, so that the amplitudes alone can be used as sufficient parameters to characterize the body centre of mass oscillations. The technique enables detection of small but consistent gait asymmetries. The walking speed strongly influences some of the symmetric gait parameters but, for normal subjects at least, the walking speed does not affect the asymmetric parameters.