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Background Many studies have detected a negative relationship between income inequality and general measures of health. However, data limitations have prevented a full understanding of whose health is impacted and in what ways.Methods In this study, we combined area-level census data with individual-level health claims data to estimate the cross-sectional association between county-level income inequality and healthcare utilisation across a range of member characteristics.Results We found that a 1 SD increase in the Gini coefficient was associated with about 5% higher medical and pharmacy costs and a 0.2 percentage-point increase in the probability of a hospital visit within the year. Income inequality was associated with higher medical costs primarily among adults with commercial insurance, more emergency department visits among children and Medicaid members, and more hospital visits among older adults, including Medicare members. By examining diagnoses attached to claims, we found that income inequality was associated with detrimental impacts on mental health, as indicated by higher spending for anxiety and depression and more emergency department visits for substance-use disorders.Conclusions Income inequality was associated with worse health across a wide range of members by age, income and insurance type, and can be considered as a risk factor by policymakers and health systems.