The NA57 experiment has been designed to study the onset of the strange baryon and antibaryon enhancements in Pb-Pb with respect to p-Be collisions, first observed by WA97 at 160 A GeV/c. NA57 has extended the WA97 measurements to investigate the evolution of strangeness enhancement over a wider centrality range and as a function of the beam energy. Results on hyperon production in central Pb-Pb collisions at 40 A GeV/c are presented and compared with those obtained at higher energy, in the same centrality range.