In this paper we study complete maximal spacelike hypersurfaces in anti-de Sitter space with either constant scalar curvature or constant non-zero Gauss-Kronecker curvature. We characterize the hyperbolic cylinders , as the only such hypersurfaces with () principal curvatures with the same sign everywhere. In particular we prove that a complete maximal spacelike hypersurface in with negative constant Gauss-Kronecker curvature is isometric to .