Otlertuzumab (TRU-016) is a humanized anti-CD37 protein therapeutic that triggers direct caspase-independent apoptosis of malignant B cells and induces antibody-dependent cell-mediated cytotoxicity. Patients with relapsed chronic lymphocytic leukaemia (CLL) received either otlertuzumab (20 mg/kg) weekly by IV infusion for two 28-day cycles then every 14 days for four 28-day cycles and IV bendamustine (70 mg/m2) on Days 1 and 2 of each cycle for up to six 28-day cycles or bendamustine alone. Thirty-two patients were treated with otlertuzumab and bendamustine and 33 with bendamustine alone. Overall response rate according to the International Workshop on Chronic Lymphocytic Leukaemia criteria was 69% in the otlertuzumab and bendamustine arm and 39% in the bendamustine alone arm (P = 0·025). Median progression-free survival (PFS) was 15·9 months in the otlertuzumab and bendamustine arm and 10·2 months in the bendamustine alone arm (P = 0·0192). There was a higher incidence of pyrexia (34% vs. 12%) and neutropenia (59% vs. 39%) with the combination but this did not result in a higher incidence of severe (grade 3/4) infections (13% vs. 27%). This combination significantly increased the response rate and prolonged the PFS over single agent bendamustine in patients with relapsed or refractory CLL.