Key Points Question How has the concentration of health care spending in the US changed by population subgroup and expenditure type between 2001 and 2018? Findings In this cross-sectional study, the overall concentration of health care expenditures remained stable, although there was a sharp increase in the concentration of spending on prescription drugs. In 2001, one-half of all expenditures on prescription drugs were concentrated in 6.0% of the US population, but by 2018, this proportion had decreased to 2.3% of the population. Meaning These findings suggest that if this trend continues, it will have implications for the minimum scale of risk-bearing and drug management needed for health insurance plans to operate efficiently, as well as the optimal cost-sharing features of insurance products.