Background: Ibrutinib is an oral, once daily Bruton’s tyrosine kinase inhibitor and is a targeted treatment that has demonstrated progression-free survival and overall survival benefits in multiple phase 3 trials across age, fitness, and high-risk markers for CLL/SLL. Despite ibrutinib established as a preferred first-line (1L) treatment option, anti-CD20 monotherapy (rituximab or obinutuzumab) has been prescribed as symptomatic or palliative therapy. This retrospective study compared outcomes between ibrutinib and anti-CD20 monotherapy for CLL/SLL patients who initiated a 1L of therapy in community oncology practice. Methods: The nationwide Flatiron Health Electronic Health Record-derived de-identified database (03/04/2016-08/31/2021) was used to select patients with CLL/SLL who had ≥2 clinic visits and started 1L therapy with ibrutinib or anti-CD20 monotherapy. Patient baseline characteristics prior to 1L initiation were compared between the two treatment groups. Propensity score-based inverse probability of treatment weighting (IPTW) was used to adjust for baseline differences between treatment groups. Kaplan-Meier (KM) analysis was used to evaluate time to next treatment (TTNT) from 1L initiation. Two subgroup analyses were performed among patients with high-risk cytogenetic markers (deletion 17p or 11q, or unmutated IGHV) and those without IGHV testing. Results: Overall, 3,226 patients were included in the analysis, of whom 2,188 (67.8%) received ibrutinib and 1,038 (32.2%) received an anti-CD20 antibody. The average age was 71.4 years for ibrutinib patients and 72.9 years among anti-CD20 users. Patients treated with ibrutinib were more likely to have high-risk cytogenetic markers (42.6% vs 27.9%). Ibrutinib-treated patients overall had a significant 70% risk reduction of initiating a subsequent treatment compared with anti-CD20 group (Table). Similar results were also found in subgroups of patients with high-risk and those without IGHV testing. Conclusions: IPTW-adjusted analysis showed that in the real-world community practice setting, treatment with ibrutinib resulted in a statistically significantly lower risk of initiating subsequent treatment than the anti-CD20 group including patients with high cytogenetic risk features and without IGHV testing.