Clinicians’ billing practices for professional services in the emergency department (ED) have come under scrutiny as the proportion of expensive high-intensity visits has grown in recent decades. Clinicians respond to payers’ criticism by citing the worsening health status of undifferentiated patients alongside increasing expectations of ED care, with few data available to disentangle these phenomena from coding practices. We performed an observational study of US treat-and-release ED visits using data from the Nationwide Emergency Department Sample. In 2006, 4.8 percent of treat-and-release ED visits exhibited high-intensity billing, and this figure rose to 19.2 percent by 2019. The proportion of visits for older patients, those with more comorbidities, and those with nonspecific but potentially serious diagnoses grew. Of the observed growth in high-intensity billing, 47 percent was expected, based on changes in administrative measures for patient case-mix and care services. Any emergency care reimbursement reform must account for growing patient complexity and an evolving role for EDs in the health care system.