Background: Insured patients who receive out-of-network care may receive a "balance bill" for the difference between the practitioner's charge and their insurer's contracted rate. In 2017, California banned balance billing for anesthesia care. This study examined the association between California's law and subsequent payments for anesthesia care. The authors hypothesized that, after the law's implementation, there would be no change in in-network payment amounts, and that out-of-network payment amounts and the portion of claims occurring out-of-network would decline., Methods: The study used average, quarterly, California county-level payment data (2013 to 2020) derived from a claims database of commercially insured patients. Using a difference-in-differences approach, the change was estimated in payment amounts for intraoperative or intrapartum anesthesia care, along with the portion of claims occurring out-of-network, after the law's implementation. The comparison group was office visit payments, expected to be unaffected by the law. The authors prespecified that they would refer to differences of 10% or greater as policy significant., Results: The sample consisted of 43,728 procedure code-county-quarter-network combinations aggregated from 4,599,936 claims. The law's implementation was associated with a significant 13.6% decline in payments for out-of-network anesthesia care (95% CI, -16.5 to -10.6%; P < 0.001), translating to an average $108 decrease across all procedures (95% CI, -$149 to -$64). There was a statistically significant 3.0% increase in payments for in-network anesthesia care (95% CI, 0.9 to 5.1%; P = 0.007), translating to an average $87 increase (95% CI, $64 to $110), which may be notable in some circumstances but did not meet the study threshold for identifying a change as policy significant. There was a nonstatistically significant increase in the portion of claims occurring out-of-network (10.0%, 95% CI, -4.1 to 24.2%; P = 0.155)., Conclusions: California's balance billing law was associated with significant declines in out-of-network anesthesia payments in the first 3 yr after implementation. There were mixed statistical and policy significant results for in-network payments and the proportion of out-of-network claims., (Copyright © 2023 American Society of Anesthesiologists. All Rights Reserved.)