One approach to assist writers in dealing with the problem of managing cognitive resources during writing is to scaffold writing by computer. Unfortunately, empirical research on computer-based writing scaffolds is quite limited. This study investigates to which extent computer-based scaffolds should be provided to support efficient prewriting. The writing environment scaffolds prewriting by instructing an adequate sequence of prewriting activities on consecutive file cards. Writers' behaviour and performance were investigated in two versions of the writing environment (lean vs. enriched version). Writing performance was further analysed with respect to students' writing strategies and motivation. The results indicate that particular motivational variables play an important role when prewriting is scaffolded by a computer. Implications of these results are discussed with regard to the restrictions and benefits of computer-based scaffolding of prewriting activities.