Patients treated at in-network facilities can involuntarily receive services from out-of-network providers, which may result in "surprise bills." While several studies report the surprise billing prevalence in emergency department and inpatient settings, none document the prevalence in ambulatory surgery centers (ASCs). The extent to which health plans pay a portion or all of out-of-network providers' bills in these situations is also unexplored. We analyzed 4.2 million ASC-based episodes of care in 2014–17, involving 3.3 million patients enrolled in UnitedHealth Group, Humana, and Aetna commercial plans. One in ten ASC episodes involved out-of-network ancillary providers at in-network ASC facilities. Insurers paid providers' full billed charges in 24 percent of the cases, leaving no balance to bill patients. After we accounted for insurer payment, we found that there were potential surprise bills in 8 percent of the episodes at in-network ASCs. The average balance per episode increased by 81 percent, from $819 in 2014 to $1,483 in 2017. Anesthesiologists (44 percent), certified registered nurse anesthetists (25 percent), and independent laboratories (10 percent) generated most potential surprise bills. There is a need for federal policy to expand protection from surprise bills to patients enrolled in all commercial insurance plans.