A method for hydroelastic analysis of high speed vessels is presented. The flexibility of the ship hull is taken into account by using a number of dry eigenmodes in addition to the 6 rigid body modes, as generalized degrees of freedom. The generalized added mass, damping and restoring matrices, as well as the generalized excitation forces, are calculated by extending the method presented by Faltinsen and Zhao (1991) to include flexible modes. Transfer functions from the hydroelastic analysis are compared with those from a conventional quasi-dynamic analysis for two idealized vessels, a monohull and a catamaran. The results indicate that springing may be a problem for large high speed vessels, but analyses of more realistic ships must be carried out before any conclusions can be drawn.