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Background: Mosunetuzumab is a first-in-class CD20x- CD3 T-cell engaging bispecific antibody that redirects T-cells to eliminate malignant B-cells. In a pivotal phase 2 study (NCT02500407), fixed-duration mosunetuzumab demonstrated a high rate of complete responses (CRs) and durable responses in patients with relapsed/refractory (R/R) follicular lymphoma (FL) and ≥2 prior therapies. Objective: To present updated data from this phase 2 study of mosunetuzumab in patients with R/R FL after 28.3 months' median follow-up. Methods: Patients with grade 1-3a FL and ≥2 prior therapies were enrolled. Intravenous mosunetuzumab was administered with step-up dosing in Cycle 1 (21-day cycles) for 8 cycles if CR by Cycle 8, and for 17 cycles if partial response or stable disease by Cycle 8. The primary endpoint was CR rate. Post hoc analyses compared investigator- assessed efficacy outcomes with mosunetuzumab versus last prior therapy, and the correlation between cytokine release syndrome (CRS) and tumor response. Results: Ninety patients were enrolled. Median age was 60 years (range: 29-90 y), and 77% of patients had stage III/ IV disease. Investigator-assessed objective response rate (ORR) and CR rate were 78% and 60% with mosunetuzumab, respectively, versus 56% and 36% with last prior therapy, respectively. Median progression-free survival (PFS) per investigator assessment was 24 months with mosunetuzumab versus 12 months with last prior therapy; 24-month PFS was 48% with mosunetuzumab. Mosunetuzumab improved median duration of response versus last prior therapy (not reached [NR] vs 12 mo, respectively), median duration of CR (NR vs 15 mo, respectively); and time-to-next therapy or death (NR vs 17 mo, respectively). CRS events, experienced by 44% of patients, were mainly (95%) grade 1/2; all CRS events resolved. Since the previous analysis, no new CRS events, or fatal, serious, or grade ≥3 adverse events were reported. ORRs were similar in patients with or without CRS events. Conclusion: Durable responses continued to be observed with mosunetuzumab. The safety profile, with predominantly low-grade CRS events, supports outpatient administration of mosunetuzumab and was consistent with previous analyses. Comparable clinical response was observed regardless of CRS occurrence. Funding: This study is sponsored by Genentech, Inc. Third-party medical writing assistance, under the direction of all authors, was provided by Emily Lynch, PhD, of Ashfield MedComms, an Inizio company, and was funded by F. Hoffmann-La Roche Ltd. [ABSTRACT FROM AUTHOR]