In this paper, I examine the impact of uninsured patients on the in-hospital mortality rate of insured heart attack patients. I employ panel data models using patient discharge and hospital financial data from California (1999–2006). My results indicate that uninsured patients have an economically significant effect that increases the mortality rate of insured heart attack patients. I show that these results are not driven by alternative explanations, including reverse causality, patient composition effects, sample selection or unobserved trends and that they are robust to a host of specification checks. The primary channel for the observed spillover effects is increased hospital uncompensated care costs. Although data limitations constrain my capacity to check how hospitals change their provision of care to insured heart attack patients in response to reduced revenues, the evidence I have suggests a modest increase in the quantity of cardiac services without a corresponding increase in hospital staff.