Using the technique of ballistic electron magnetic microscopy, we have studied the spin-asymmetry of transmission of hot electrons in Fe, for which a recent ab initio calculation has shown that the inelastic lifetime is similar for majority and minority spin. Nevertheless, using a spin-valve structure of Ni81Fe19/Au/Fe, we find that the attenuation length of hot electrons in Fe at 1.2–1.6 eV is a factor of 4 larger for the majority spin. We argue that this large spin-asymmetry arises from the spin dependence of the group velocity, rather than the lifetime.