For patients with progressive or recurrent glioblastoma there is no standard therapy. One strategy is re-exposure to temozolomide in a higher dose. This increase in dosing can be done by 2 regimens. Aim of this study is to compare these 2 dosing regimens concerning toxicity. In study arm A patients receive temozolomide for one week, followed by a week without temozolomide. In study arm B patients receive temozolomide for three weeks, followed by a week without temozolomide. The regimen that is less toxic will be selected for further evaluations.