Importance: Although there are increasing differences in health policy and population health across US states over the past decade, little is known about how US states compare with other countries on avoidable mortality.

Objective: To compare trends in avoidable mortality across US states and countries in the European Union (EU) and the Organisation for Economic Co-operation and Development (OECD).

Design, setting, and participants: Retrospective, population-based, repeated cross-sectional study comparing changes in avoidable mortality among decedents aged 0 to 74 years in 50 US states (and Washington, DC) and 40 high-income countries between 2009 and 2021. Data analysis was conducted from May to July 2024.

Main outcomes and measures: Avoidable mortality comprising both preventable deaths related to prevention and public health and treatable deaths related to timely and effective health care treatment.

Results: Between 2009 and 2019, total avoidable mortality increased in all US states (median [IQR], 29.0 [20.1 to 44.2] deaths per 100 000 people), while it decreased in most comparator countries (-14.4 [-28.4 to -8.0]). During this period, variation in avoidable mortality widened across US states (2009: median [IQR], 251.1 [228.4 to 280.4]; 2019: 282.8 [249.1 to 329.5]), but narrowed in comparator countries (2009: 201.5 [166.2 to 320.8]; 2019: 187.1 [152.0 to 298.2]). During the COVID-19 pandemic (2019-2021), avoidable mortality increased for all US states (median [IQR], 101.5 [64.7 to 143.1]) and comparator countries (25.8 [9.1 to 117.7]). The states and countries that experienced the greatest increase in avoidable deaths during the COVID-19 period were those with the highest baseline avoidable mortality (Pearson ρ = 0.86; P < .001). Health spending and avoidable mortality have a consistent, negative, and significant association among comparator countries (2019: Pearson ρ = -0.7; P < .001) but no statistically significant association within US states (2019: Pearson ρ = -0.12; P = .41).

Conclusions and relevance: This cross-sectional study found that the stark contrast in avoidable mortality trends between all US states compared with EU and OECD countries suggests that broad, systemic factors play a role in worsening US population health. While other countries appear to make gains in health with increases in health care spending, such an association does not exist across US states, raising questions regarding US health spending efficiency.