We prove existence and multiplicity of solutions for the problem
$$\left\{ {\matrix{ {\Delta ^2u + \lambda \Delta u = \vert u \vert ^{2*-2u},{\rm in }\Omega ,} \hfill \hfill \hfill \hfill \cr {u,-\Delta u > 0,\quad {\rm in}\;\Omega ,\quad u = \Delta u = 0,\quad {\rm on}\;\partial \Omega ,} \cr } } \right.$$ where $\Omega \subset {\open R}^N$, $N \ges 5$, is a bounded regular domain, $\lambda >0$ and $2^*=2N/(N-4)$ is the critical Sobolev exponent for the embedding of $W^{2,2}(\Omega )$ into the Lebesgue spaces.