The biodegradation of ultrahigh-purity Mg (XHP Mg, Fe ≈ 2 ppm) was tested in vitro in NaHCO3/CO2-buffered simulated body fluid and in vivo in the femur of rats. The in vitro degradation rate, which was evaluated using an also here described newly designed hydrogen evolution testing setup, is with ≈10 ± 3 μm/y very low and shows very good agreement with in vivo data. The results for XHP Mg are also compared with in vitro tests on high-purity Mg (Fe ≈ 37 ppm) in the as-cast and annealed states. The less pure specimens exhibit significantly higher degradation rates due to the formation of Fe-containing precipitates during casting and annealing.