  We use a relative Fourier-Mukai transform on elliptic K3 surfaces $X$ to
describe mirror symmetry. The action of this Fourier-Mukai transform on the
cohomology ring of $X$ reproduces relative T-duality and provides an
infinitesimal isometry of the moduli space of algebraic structures on $X$
which, in view of the triviality of the quantum cohomology of K3 surfaces, can
be interpreted as mirror symmetry.
