Mantle cell lymphoma (MCL) is an aggressive non-Hodgkin lymphoma with a poor prognosis following first relapse. We present a subgroup analysis of an open-label phase II trial investigating the efficacy and safety of lenalidomide in patients with relapsed or refractory MCL. Oral lenalidomide 25 mg was self-administered once daily on days 1-21 every 28 d for up to 52 weeks, according to tolerability or until disease progression. The primary endpoint was overall response rate (ORR) and secondary endpoints were duration of response, progression-free survival (PFS) and safety. Among 15 patients with MCL with a median disease duration of 5.1 years and a median of four prior treatments, the ORR was 53%. Three patients (20%) had a complete response and 5 (33%) had a partial response. The median duration of response was 13.7 months and median PFS was 5.6 months. Four of five patients who relapsed after transplantation and two of five patients who previously received bortezomib responded to lenalidomide. The most common grade 4 adverse event was thrombocytopenia (13%) and the most common grade 3 adverse events were neutropenia (40%), leucopenia (27%) and thrombocytopenia (20%). In conclusion, oral lenalidomide monotherapy is well tolerated and active in relapsed or refractory MCL.