U.S. Supreme Court Justices said Monday they would consider whether to keep alive the largest employment discrimination lawsuit in U.S. history, a case that claims the owners of Wal-Mart pay women less than men and promote women less frequently.

The dispute could end in billions of dollars in back pay for 500,000 to 1.5 million women who work or once worked at Wal-Mart Stores Inc. Six women first filed the lawsuit in federal court in 2001.

Wal-Mart officials are appealing a ruling by the 9th U.S. Circuit Court of Appeals that the lawsuit could go to trial, arguing that allowing the large number of claims to go forward would set off an avalanche of similar class-action lawsuits in other regions overseen by the 9th Circuit.

The lawyers representing the women suing Wal-Mart say there have been only eight such suits nationwide—and none within the 9th Circuit—since the first appeals court ruling in favor of the women nearly four years ago.

Wal-Mart officials also said the women should not be allowed to join together in the lawsuit because each outlet operates as an independent business. Since they don’t have a companywide policy of discrimination, Wal-Mart officials said women alleging gender bias should file individual lawsuits against individual stores.

The plaintiffs contend the company was aware it lagged behind other employers in opportunities for women and that Wal-Mart officials impose uniform rules and tight controls over their stores.

In its latest decision in April, the appeals court voted 6-5 in favor of the plaintiffs. Judge Michael Daly Hawkins said that the number of women involved is large, but “mere size does not render a case unmanageable.”

Judge Sandra Ikuta’s dissent said the women employees failed to present proof of widespread discrimination. Therefore, Ikuta said, “there is nothing to bind these purported 1.5 million claims together in a single action.”

The case will be argued in the spring.

The Associated Press contributed to this report.