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.Comment: 15 pages, AMS-LaTeX v1.2. Final version to appear in Commun. Math.
Phy