A measurement of strange baryon and antibaryon production in Pb-Pb collisions has been carried out by the NA57 experiment at the CERN SPS, with 40 and 158 AGeV/c beam momentum. Results on Λ, Ξ and Ω hyperon yields at mid-rapidity in the most central 53% of Pb-Pb collisions at 40 AGeV/c are presented and compared with those obtained at higher energy, in the same collision centrality range. The Λ and Ξ^- yields per unit rapidity stay roughly constant while those of Ω^-, Λ̄, Ξ̄⁺ and Ω̄⁺ increase when going to the higher SPS energy. Hyperon yields at the SPS are compared with those from the STAR experiment in s_NN=130 GeV Au-Au collisions at RHIC.