The law requires insurance companies and other businesses to notify customers who are charged more because of their credit ratings.
In a unanimous decision, the justices modified a standard of the 9th U.S. Circuit Court of Appeals in San Francisco that the insurance industry said could have subjected it to billions of dollars in punitive damages for failing to notify customers.
Thirteen state insurance commissioners said that a lower threshhold for proving liability would motivate compliance with the law.
The two companies in the case are Safeco Insurance Company of America and Geico General Insurance Co.