  In this paper, we prove that the Brezis-Nirenberg problem -\Delta u =
|u|^{p-1}u+\epsilon u in \Omega; u=0 on \partial \Omega where \Omega is a
symmetric bounded smooth domain in R^N, N\geq 7 and p = (N+2)/(N-2), has a
solution with the shape of a tower of two bubbles with alternate signs,
centered at the center of symmetry of the domain, for all \epsilon > 0
sufficiently small.
