Structures and energies of cyclic Si5H5+ forms have been studied ab initio at the MP2/6-31G(d) level. Surprisingly, the C-2 upsilon pentasila[1.1.1]propellanyl cation (13) has been found to be the most stable isomer. The energies of two pyramidal structures (6 and 35) and two four-membered cyclic isomers (30 and 31) are within a 5 kcal/mol range of 13. In contrast with the case of (CH)(5)(+), neither singlet nor triplet planar five-membered (SiH)(5)(+) rings are minima. The nonplanar five-membered C-s pentasilacyclopentadienyl cation (20) is 21 kcal/mol higher in energy than 13 but has a small positive (favorable) homodesmotic stabilization energy (SE = 2 kcal/mol : 20 + Si5H10 --> Si5H7+ + Si5H8) in contrast to its planar carbon analog (9) (SE= -35 kcal/mol : 9 + C5H10 --> C5H7+ + C5H8).