Hydrogen can have a pronounced effect on the mechanical properties of titanium (Ti) and Ti alloys, such as deformation, fracture, and fatigue life. The delayed fracture of medical Ti alloy devices is commonly caused by hydrogen embrittlement. Experimentally, we confirmed that the degradation in the performance of Ni-Ti superelastic alloy is caused by the hydrogen absorption related with specific factors in vivo; i.e., hydrogen embrittlement is related to such factors as sustained stress, in vivo environment (pH and potential), and surface chemistry. In our research a thermal desorption gas spectrometer (TDS) was used to measure the hydrogen content in the alloys without fusion. The measured hydrogen concentration showed clearly that the hydrogen concentration affects considerably the degradation of the mechanical properties. It is unclear how the alloy can naturally absorb hydrogen under biological circumstances. However, it is suspected that the hydrogen absorption is related to galvanic currents, fretting corrosion, hydrogen ions in a biological fluid, and changes in pH under biostructures, biofilms, and tumors. It may lead to the degradation of their mechanical properties by hydrogen absorption in vivo. Also the processes of medical Ti alloy devices, such as pickling, are associated with their hydrogen content. Thus, the estimation of the hydrogen absorption in vivo in relation to surface features is required to evaluate its effect on such properties as fatigue and the service life of the Ni-Ti alloys used as medical devices.