  The leading radiative correction to the Casimir energy for two parallel
penetrable mirrors is calculated within QED perturbation theory. It is found to
be of the order $\alpha$ like the known radiative correction for ideally
reflecting mirrors from which it differs only by a monotonic, powerlike
function of the frequency at which the mirrors become transparent. This shows
that the $O(\alpha^2)$ radiative correction calculated recently by Kong and
Ravndal for ideally reflecting mirrors on the basis of effective field theory
methods remains subleading even for the physical case of penetrable mirrors.
