  We study the inverse spectral problem for periodic Schr\"odinger opera\-tors
of kind $- \frac{1}{2} \hbar^2 \Delta_x + V(x)$ on the flat torus $\Bbb T^n :=
(\Bbb R / 2 \pi \Bbb Z)^n$ with potentials $V \in C^{\infty} (\Bbb T^n)$. We
show that if two operators are isospectral for any $0 < \hbar \le 1$ then they
have the same effective Hamiltonian given by the periodic homogenization of
Hamilton-Jacobi equation. This result provides a necessary condition for the
isospectrality of these Schr\"odinger operators. We also provide a link between
our result and the spectral limit of quantum integrable systems.
