Abstract. We study the stability of immersed compact constant mean curvature (CMC) surfaces without boundary in some Riemannian 3-manifolds, in particular the Riemannian product spaces S 2 × R and H 2 × R. We prove that rotational CMC spheres in H 2 × R are all stable, whereas in S 2 × R there exists some value H 0 ≈ 0.18 such that rotational CMC spheres are stable for H ≥ H 0 and unstable for 0 < H < H 0 . We show that a compact stable immersed CMC surface in S 2 × R is either a finite union of horizontal slices or a rotational sphere. In the more general case of an ambient manifold which is a simply connected conformally flat 3-manifold with nonnegative Ricci curvature we show that a closed stable immersed CMC surface is either a sphere or an embedded torus. Under the weaker assumption that the scalar curvature is nonnegative, we prove that a closed stable immersed CMC surface has genus at most three. In the case of H 2 × R we show that a closed stable immersed CMC surface is a rotational sphere if it has mean curvature H ≥