Patients benefitting from BIG treatment will enter the maintenance phase of the trial. Maintenance treatment will be continued if no unacceptable toxicity occurs until three months after negativity of minimal residual disease [MRD] is achieved in peripheral blood in patients with (clinical) complete response (CR) or (clinical) incomplete complete response [CRi] (confirmed by 2 consecutive testings of MRD within 3 months), progression of CLL, start of a subsequent therapy or up to 8 cycles of maintenance (each cycle with a duration of 84 calendar days = 3 months), whichever occurs first.