The s-wave isoscalar amplitude for pi pi scattering in a nuclear medium is evaluated using a nonperturbative unitary coupled channels method and the standard chiral Lagrangians. The method has proved successful to describe the pi pi properties in the scalar-isoscalar channel up to 1.2 GeV, giving rise to poles in the t matrix for the f(0)(980) and the sigma. The extension of the method to the nuclear medium implies not only the renormalization of the pions in the medium, but also the introduction of interaction terms related to contact terms in the pi N --> pi pi N interaction. Off-shell effects are also shown to be important leading to cancellations which reduce the coupled channel integral equations to a set of algebraic equations. As the density increases we find a reduction of strength below the a region and a certain accumulation of strength at energies around pion threshold. Our results, based on chiral Lagrangians, provide similar results to those obtained with phenomenological models which impose minimal chiral constraints.