This paper examines the equilibrium effects of insurance contracts on healthcare markets using a mechanism design framework. A population of risk-averse agents with preferences as in Yaari (1987) face the risk of developing an illness of unknown severity, which can be treated in a competitive hospital services market at the prevailing market price. After privately observing their health risk, but before learning their sickness level, agents have the option to purchase insurance from a monopolistic provider. Insurance contracts specify premiums, out-of-pocket costs (OPCs), and hospital service coverage, thus determining demand and price in the downstream hospital market through a market-clearing condition. Our first main result shows that optimal insurance contracts take a simple form: agents can choose between full hospital coverage with a high OPC or restricted coverage with a low OPC. This highlights a novel form of under-insurance (rationing or restricted access to healthcare services) emerging purely due to the insurer's attempt to control his price impact. Our second key result illustrates the nuanced effect of price impact on the amount of insurance provided. Higher healthcare prices increase insurer payouts but also worsen agents' outside options, making them more willing to pay for insurance ex ante. The net effect of these forces determines whether insurance provision exceeds or falls short of a price-taking benchmark.