Background: Rituximab is a chimeric anti-CD20 monoclonal antibody therapy, used primarily for treating chronic lymphocytic leukemia (CLL) and non-Hodgkin’s lymphoma (NHL). Rituximab-abbs, the first rituximab biosimilar, was approved in the UK in 2017 and was expected to significantly reduce drug acquisition costs, but there is a lack of real-world evidence regarding patient outcomes with rituximab-abbs. This non-interventional study assessed the real-world effectiveness and tolerability of rituximab-abbs and rituximab in treatment-naive patients with CLL or NHL. Methods: Anonymized data on patient characteristics, response to treatment, healthcare resource utilization and costs were abstracted retrospectively via an online physician survey. UK-registered hematologists and oncologists reported on randomly selected patients aged ³18 years from four cohorts with documented CLL or NHL, who had received rituximab-abbs or rituximab as first-line immunotherapy (between January 1, 2018 and June 30, 2019). Patient outcomes data were provided from first treatment to the last date of follow-up available in the medical records. Results: In total, 46 physicians abstracted data from 201 patient charts. Demographic profiles of the cohorts were similar. For both treatments, the overall response rate (ORR) was very high for patients with CLL or NHL (Table) along with rates for six-month progression-free survival (96–98% across cohorts) and one-year survival (98–100% across cohorts). Most patients did not experience a grade ≥3 adverse event during treatment (54–66% across cohorts); the most common grade ≥3 adverse events were neutropenia, fatigue, anemia and infusion reactions. Healthcare resource utilization was similarly high across cohorts, driven by drug costs, diagnostic testing, oncologist office visits, and day case hospital admissions. Mean annual savings of approximately £1,000 per patient were seen with rituximab-abbs, attributable to first-line treatment costs. Conclusions: The originator (rituximab) and biosimilar (rituximab-abbs) products yielded comparable efficacy and tolerability in treating CLL and NHL in routine UK clinical practice, with rituximab-abbs demonstrating cost-savings. These findings should inform decision-makers on the potential for cost reductions where the biosimilar rituximab-abbs is available as a treatment alternative.