For a subgroup H of an alternating or symmetric group G, we consider the Möbius number μ(H, G) of H in the subgroup lattice of G. Let b m (G) be the number of subgroups H of G of index m with μ(H, G) = 0. We prove that there exist two absolute constants α and β such that for any alternating or symmetric group G, any subgroup H of G and any positive integer m we have b m (G) m α and |μ(H, G)| |G : H| β .