For elderly patients with mantle cell lymphoma (MCL), there is no defined standard therapy. In this multicenter open-label phase I/II trial we evaluated the addition of lenalidomide (LEN) to rituximab-bendamustine (R-B) as first-line treatment to elderly MCL patients. Patients >65 years with untreated MCL, stage II-IV were eligible for inclusion. Primary endpoints were maximally tolerable dose (MTD) of LEN, and progression-free survival (PFS). Patients received six cycles q4w of L-B-R (L D1-14, B 90 mg/m(2) iv D1-2 and R 375 mg/m(2) iv D1) followed by single LEN (D1-21, q4w, cycles 7-13). 51 patients (median age 71 years) were enrolled 2009-2013. In phase I, the MTD of LEN was defined as 10 mg in cycles 2-6, and omitted in cycle 1. After six cycles, the complete remission rate (CRR) was 64% and 36% were MRD negative. At a median follow-up time of 31 months, median PFS was 42 months and 3 year overall survival was 73%. Infection was the most common non-hematological grade 3-5 event and occurred in 21 (42%) patients. Opportunistic infections occurred in three patients; 2 PCP and 1 CMV retinitis. Second primary malignancies (SPM) were observed in eight patients (16%). LEN could safely be combined with R-B, when added from the second cycle in patients with MCL, and was associated with a high rate of CR and molecular remission. However, we observed a high degree of severe infections and an unexpected high number of SPMs which may limit its use. http://clinicaltrials.gov: NCT00963534.