A goal of many physiatric interventions is to improve biomechanical walking efficiency. Thus, a tool that helps assess this efficiency, independent of cardiac, pulmonary, psychologic, or other nonbiomechanical factors, would be useful. Currently used methods to measure efficiency, including comfortable walking speed, are not specific to biomechanical variables. A potential tool, the biomechanical efficiency quotient (BEQ), which uses three variables-average stride length, vertical displacement of the trunk during walking, and sacral height during standing-is proposed and preliminarily tested. This quotient is based on Saunders, Inman, and Eberhart's theories and on a prior study in able-bodied subjects. The BEQ was computed in 20 consecutive patients with neurologically based gait disability referred for gait laboratory evaluation who subjectively reported that one or two ankle-foot-orthoses (AFOs) reduced the effort necessary to walk. The quotient was calculated with and without the AFO(s) by dividing the average vertical displacement of the sacrum, which was measured with an optoelectronic system, and by a predicted displacement, which was based on the patient's sacral height and average stride length. The mean BEQ with the AFO(s) (6.3 ± 4.4) was significantly less than the mean BEQ without the AFO(s) (9.7 ± 7.1);P= 0.005. Furthermore, the BEQ was less with the AFO(s) compared with trials without the AFO(s) in all subjects. Percent change in BEQ with the AFO(s) (26.8 ± 19.6) correlated with percent change in comfortable walking velocity (24.8 ± 31.8),r= 0.73,P< 0.001, across all subjects. The BEQ may be useful in specifically assessing the effect on biomechanical efficiency of physiatric interventions, despite variable nonbiomechanical factors. An instrument to measure vertical trunk displacement during walking outside of the gait laboratory would be extremely useful for further necessary longitudinal studies.