In the 1984 case Metropolitan Life Insurance Co. v. Massachusetts, the U.S. Supreme Court upheld a Massachusetts law requiring minimum mental health care benefits in any insurance policy issued within the state. The court ruled that this law was not preempted by the Employee Retirement Income Security Act (ERISA), which regulates employee benefit plans but does not mandate specific benefits to be included in such plans. In its decision, the court emphasized that states have traditionally held broad power to regulate insurance and ERISA did not intend to displace these powers unless they directly conflicted with federal regulations or prevented uniform administrative practice among states regarding employee benefit plans.

In the dissenting opinion for Metropolitan Life Insurance Co. v. Massachusetts, Justice O'Connor argued that the majority's decision to uphold a state law requiring certain minimum mental health care benefits in employee insurance plans was inconsistent with previous rulings on ERISA preemption. She contended that this ruling would open up a "Pandora’s box" of potential state regulations over benefit structures and could undermine uniform national administration of employee benefit plans, which is one of ERISA's main objectives. Furthermore, she disagreed with the majority's interpretation that such mandated-benefit laws fell under 'insurance regulation,' thus exempt from federal preemption by ERISA; instead, she viewed them as indirect attempts to regulate plan terms and conditions - an area explicitly preempted by ERISA.