The federal Affordable Care Act (ACA) imposed many new regulations on the health insurance marketplace, including a minimum medical loss ratio (MLR) requirement. However, several states had minimum MLR regulations in place prior to the enactment of the ACA. In this paper, we examine whether insurers operating in states with pre-ACA MLR regulations were better able to adapt to the new MLR requirements imposed by the ACA. We find evidence that state MLR requirements have adversely affected insurer market share. We also find that insurers operating in states without existing MLR requirements experienced decreases in market share relative to insurers operating in states with MLR requirements. Finally, there is evidence of a differential effect of the state and federal MLR requirements based on the extent of market share, with smaller insurers being more adversely affected.