  We derive a general formula for the correlation function of two identical
particles with the inclusion of multiple elastic scatterings in the medium in
which the two particles are produced. This formula involves the propagator of
the particle in the medium. As illustration of the effect we apply the formula
to the special case where the scatterers are static, localized 2-body
potentials. In this illustration both $R^2_{\rm side}$ and $R^2_{\rm out}$ are
increased by an amount proportional to the square of the spatial density of
scatterers and to the differential cross section. Specific numbers are used to
show the expected magnitude of the rescattering effect on kaon interferometry.
