This is a non-randomized, multicenter, open-label, single-arm Phase I/II study in patients with previously untreated CD20-positive CLL. Phase I of the study will evaluate the maximal tolerated lenalidomide dose level in combination with fludarabine/rituximab chemoimmunotherapy in 10 patients. Phase II will determine efficacy using the combination in the previously defined maximal tolerated dose. Both phases will be followed by a maintenance phase evaluating the tolerability and possibility to further improve response quality.