In the case of Rush Prudential HMO, Inc. v. Debra C. Moran et al., 2001, the US Supreme Court ruled in favor of Debra C. Moran who was seeking coverage for a surgery recommended by her doctor but denied by her health maintenance organization (HMO), Rush Prudential HMO, Inc.. The Illinois law required independent medical review when an HMO denies treatment suggested by a physician and this state-mandated external review process determined that the procedure should be covered under Moran's insurance plan with Rush Prudential. However, Rush argued that such state laws were preempted by federal Employee Retirement Income Security Act (ERISA) which governs private employer-sponsored group health plans and does not mandate an external review process or guarantee payment for medically necessary services as defined by a treating provider. The court held in a 5-4 decision that ERISA did not preclude states from providing additional protections to patients beyond those provided under federal law and thus upheld the ruling of lower courts ordering Rush to cover cost of Ms.Moran’s surgery.

In the dissenting opinion for Rush Prudential HMO, Inc. v. Debra C. Moran et al., Justice Clarence Thomas, joined by Chief Justice William Rehnquist and Justices Antonin Scalia and Anthony Kennedy, argued that Illinois' independent review statute was preempted by the Employee Retirement Income Security Act (ERISA). They contended that ERISA's civil enforcement provisions were intended to be exclusive remedies for participants in employee benefit plans covered under ERISA. The majority's interpretation of these provisions allowed states to supplement or supplant them with their own laws which contradicted Congress’s intent when it enacted ERISA - a uniform regulatory regime over employee benefit plans. Furthermore, they disagreed with the majority's view that an HMO makes a "medical necessity" determination as part of its fiduciary duties under ERISA rather than as part of its treatment decisions not governed by federal law.