6601 Background: There is poor compliance among cancer centers with price transparency rules set forth by the CMS in 2021. Service charges higher than what a hospital reasonably expects to be reimbursed may both prevent competitive price shopping by the consumer and result in catastrophic financial toxicity for small payors and under/uninsured individuals. We investigated variability in the charged and reimbursed payments for pembrolizumab infusion within one metro area utilizing Medicare claims. Methods: Medicare claims data from 2016 through 2021 for all pembrolizumab (HCPCS J9271) infusions in a hospital outpatient department were selected from the Definitive Healthcare claims database for medical oncology providers in New York City (NYC). Average charges, reimbursements, and total claims were obtained for the 6 largest prescribers by volume. Annual percentage changes (APC) were calculated to evaluate trends during study period. Prices are presented as inflation adjusted numbers (charged and reimbursed; 2021 dollars). Analyses were performed in Excel (Microsoft Corp). Results: Our analysis included 5 NCI-designated cancer centers (NCI-A, NCI-B, NCI-C, NCI-D) of which 1 was PPS (Prospective Payment System)-exempt (NCI-PPS). One hospital was non-NCI and 4 hospitals were part of the discounted 340B Drug Pricing Program (NCI-A, NCI-B, NCI-D, Non-NCI). From 2016 to 2021, total Medicare claims increased 793 to 1537, and total payments to all centers increased from $5.4M to $16.3M. NCI-PPS comprised 61% of claims, and 65% of total payments in 2021. Average charge for pembrolizumab in 2021 ranged from $20,130 (NCI-PPS) to $155,825 (NCI-D/340B). During study period, APC ranged from +1.7% (NCI-A/340B) to +25.4% (NCI-D/340B). Despite wide variability in charged amounts for Medicare claims, re-imbursed amounts were similar across centers (range: $6227-9623, 2021). Conclusions: There is significant variability in charges for pembrolizumab in Medicare claims with price increases outpacing the annual rate of inflation. Within NYC, the highest charges were from a 340B hospital; NCI-PPS had higher volume and total payments but consistently lower charged prices each year. Our study highlights extreme price variability, and has policy implications for price transparency requirements and uniform rate setting to establish payment levels and control rate of annual growth. [Table: see text]