A stent-graft prosthesis 500 includes a fabric sleeve 512 supported by a series of undulating, saddle shaped ring stents 518, each defining a plurality of peaks 520 and valleys or troughs 522. The peaks 520 and valleys 522 of at least one of the ring stents 518 are further arranged such that they are angularly offset with respect to the peaks 520 and valleys 522 of an adjacent ring stent 518. In one particular arrangement the stents 518 are arranged as mutually aligned pairs, wherein successive pairs of adjacent stents 518 are angularly offset from the preceding pair of ring stents 518. In another arrangement multiple series of ring stents may be interposed such that at least one stent in a first series of stents S1-Sn is located between two stents in another series of stents S°1-S°n, the angular offset of stents in the first series S1-Sn being different from the angular offset of stents in the other series S°1-S°n. Some ring stents 518 across or within series may have peaks 520 and valleys 522 that are however aligned.
