Ten established transfemoral amputees completed biomechanical tests after they had been fitted with a new prosthetic knee joint (test sessions: a few hours and 3 months after fitting). The required adaptation phase after fitting of a new prosthesis is frequently discussed. This phase is considered to be completed when the test result is no longer influenced by the patients’ learning processes. The study contributes to the understanding of the adaptation effects experienced prosthesis users require to master various motion patterns after being fitted with functionally new prosthetic knee joints. Biomechanical parameters (ground reaction forces, joint moments, and joint angles at the knee and the hip) were measured for level walking as well as ascending and descending ramps and stairs. For level walking, walking on ramps, and descending stairs, relevant differences in the parameters between the two tests could not be identified. For ascending stairs, the results of both tests showed obvious differences. The functions of a newly fitted prosthetic component can be intuitively used after a few hours of adaptation time if the motion patterns required are similar to that of the previous fitting. If specific functions require the learning of a motion, the learning processes will require a longer adaptation period.