The purpose of this study was to investigate whether the increased energy cost of amputee gait could be accounted for by an increase in the mechanical work dissipated during the step-to-step transition in walking. Eleven transtibial amputees (AMP) and 11 age-matched controls (CO) walked at both comfortable (CWS) and fixed (FWS, 1.3m/s) walking speed, while external mechanical work of each separate leg and metabolic energy consumption were measured. At FWS the metabolic energy consumption (E(met)) was significantly higher in AMP compared to CO (3.34 Jkg(-1)s(-1) vs. 2.73 Jkg(-1)s(-1)). At CWS, no difference in energy consumption was found (3.56 Jkg(-1)s(-1) vs. 3.58 Jkg(-1)s(-1)) but CWS was significantly lower in AMP compared to CO (1.35 ms(-1) vs. 1.52 ms(-1)). In conjunction with the higher E(met) at FWS, the negative work generated by the intact leading leg for the step-to-step transition in double support was significantly higher for AMP than CO at FWS. A moderate though significant correlation was found between negative mechanical power generated during the step-to-step transition and metabolic power (CWS: r=-0.56, p=0.007; FWS: r=-0.50, p=0.019). Despite the difference in negative work during the step-to-step transition, the total absolute mechanical work over a stride did not differ between groups. This could possibly be attributed to exchange of internal positive and negative work during single support, which remains unnoticed in the external work calculations. It was concluded that the increased mechanical work for the step-to-step transition from prosthetic to intact limb contributes to the increased metabolic energy cost of amputee walking.