The prognosis after retreating with high-dose melphalan with stem cell support after first relapse after high-dose treatment is dependent on the time to first relapse. Bortezomib can increase chemosensitivity of e.g. melphalan. The trial aims at determining the toxicity of adding bortezomib to high-dose melphalan with stem cell support and evaluating whether the time to a second relapse can be prolonged.