Background: Provider experience, or clinical volume, is associated with improved outcomes in many complex healthcare settings. Despite increased complexity of anticancer therapies, studies evaluating physician-level experience and cancer treatment outcomes are lacking. Methods: A population-based study was conducted of older adults (aged $66 years) diagnosed with B-cell non-Hodgkin's lymphoma in 2004 through 2011 using SEER-Medicare data. Analysis focused on outcomes in patients receiving rituximab, the first approved monoclonal anticancer immunotherapy. We hypothesized that lower physician experience using rituximab and managing its infusion-related reactions would be associated with early treatment discontinuation. A 12-month look-back from each initiation of rituximab was used to categorize physician volume (0, 1-2, or $3 initiations per year). Modified Poisson regression was used to account for provider-level correlation and estimated relative risk (RR) of early rituximab discontinuation (,3 cycles within 180 days of rituximab initiation). Cox proportional hazards were used to measure the impact of rituximab discontinuation on survival. Results: Among 15,110 patients who initiated rituximab with 2,684 physicians, 7.6% experienced early rituximab discontinuation. Approximately onefourth of patients (26.1%) initiated rituximab with a physician who had no rituximab initiations during the preceding 12 months. Compared with patients treated by physicians who had $3 rituximab initiations in the prior year, those treated by physicians without initiations were 57% more likely to experience early discontinuation (adjusted RR [aRR], 1.57; 95% CI, 1.35-1.82; P,.001 for 0 vs$3, and aRR, 1.19; 95% CI, 1.03-1.37; P5.02 for 1-2 vs $3). Additionally, rituximab discontinuation was associated with higher risk of death (adjusted hazard ratio, 1.39; 95%CI, 1.28-1.52; P,.001). Conclusions: Lower oncologist experience with rituximab was associated with increased risk of early rituximab discontinuation in Medicare beneficiaries with non-Hodgkin's lymphoma. Physician-level volume may be an important factor in providing high-quality cancer care in the modern era.