Background: While disparities in hospital care outcomes based on ethnicity or socioeconomic status have been documented in some nations, it is unclear to what extent this phenomenon occurs in high-income countries like Korea with relatively low income inequality. Methods: We examined in-hospital mortality rates for wealthy and poor patients in South Korea for five common medical conditions. We measured in-hospital mortality rates and examined within-hospital differences (wealthy and poor patients receive different care at the same institution) as well as between-hospital differences (wealthy and poor patients receiving care at different institutions) across socioeconomic status. We built multivariable models that adjusted for risk and further adjusted our analyses for hospital characteristics including a hospital's overall mortality ranking. Results: There were 127 438 patients with one of the five conditions examined nested in 66 Korean hospitals. We found moderate differences in mortality rates between wealthy and poor patients across four of the five conditions. These mortality differences were largely attributable to differences in mortality rates for wealthy patients compared with those for poor patients within the same hospital (within-hospital variation) while a relatively small portion of these disparities were attributable to mortality difference between hospitals where wealthy and poor patients seek care (between-hospital variation). For example, we estimate that improving care for poor patients in the same hospital can eliminate 86% of the disparities for acute myocardial infarction outcomes, while ensuring equal access to low mortality hospitals would reduce only 16% of the disparities in outcomes. Conclusions: This study suggests that to reduce socioeconomic disparities in hospital mortality, interventions that target within-hospital effects may be more effective than interventions targeting between hospital effects.