Lower extremity amputees have to cope with many activities in everyday life that are substantially more difficult than walking on level ground, and such demands require a high degree of functionality from their prosthetic components. The present study is a biomechanical evaluation (kinematics, kinetics and EMG) of stair ascent and descent in a group of eight transtibial amputees (mean (standard deviation): age 51(14) years, height 176(7)cm, mass 88(19)kg); a group of 12 transfemoral amputees (age 37(8) years, height 182(7)cm, mass 83(7)kg) fitted with the electronically controlled C-LEG knee joint system; and a group of 12 able bodied persons (age 30(10) years, height 174(12)cm, mass 69(12)kg). During stair descent the transfemoral amputees presented a strong reduction of the prosthetic ankle moments (0.11Nm/kg) compared to transtibial amputees (0.93Nm/kg) and control subjects (1.26Nm/kg). Loading of the prosthetic knee joint in the transfemoral amputees more closely resembles the loading seen in the control population when compared to transtibial amputees (mean maximum flexion moment: controls 1.31Nm/kg, transfemoral amputees 1.00Nm/kg, transtibial amputees 0.50Nm/kg). Overload of the contralateral limb is more prominent in the transfemoral amputee than in the transtibial amputee. During stair ascent, the transtibial amputee presents a significant reduction of the knee flexion moment compared to the controls (mean maximum flexion moment: transtibial amputees 0.28Nm/kg, controls 1.31Nm/kg). These differences correlate with a change in the muscle activity of the knee extensor and hamstring muscles. The results also show adaptations in motor strategies during stair negotiation, for those with the partial loss of a lower limb due to the functional limits of current prosthetic components. The present data may contribute to a further enhancement of the efficiency of prosthetic feet and knee joints.