Net metabolic cost of walking normalized by body mass, BM, (C_W.BM^-1, in J.kg^-1.m^-1) is greater in obese than in normal-weight individuals, and biomechanical differences could be responsible for this greater net metabolic cost. We hypothesized that in obese individuals, greater mediolateral body center of mass (COM) displacement and lower recovery of mechanical energy could induce an increase in the external mechanical work required to lift and accelerate the COM, and thus in net C_W.BM^-1. Body composition and standing metabolic rate were measured in 23 obese and 10 normal-weight adolescents. Metabolic and mechanical energy costs were assessed while walking along an outdoor track at four speeds (0.75-1.50 m.s^-1). Three-dimensional COM accelerations were measured by means of a tri-axial accelerometer and gyroscope, and integrated twice to obtain COM velocities, displacements, and fluctuations in potential and kinetic energies. Lastly, external mechanical work (J.kg^-1.m^-1), mediolateral COM displacement and the mechanical energy recovery of the inverted pendulum were calculated. Results: Net C_W.BM^-1 was 25% higher in obese than in normal-weight subjects on average across speeds, and net C_W.BM^-0.67 (J.kg^-0.67.m^-1) was significantly related to percent body fat (r²=0.46). However, recovery of mechanical energy and the external work performed (J.kg^-1.m^-1) were similar in the two groups. The mediolateral displacement was greater in obese subjects and significantly related to percent body fat (r²=0.64). The mediolateral COM displacement, likely due to greater step width, was significantly related to net C_W.BM^-0.67 (r²=0.49). Conclusions: We speculate that the greater net C_W.BM^-0.67 in obese subjects may be partially explained by the greater step-to-step transition costs (W_int,dc) associated with wide gait during walking.