Wal-Mart will face a lawsuit claiming pay discrimination against more than a million female US employees after a court approved the action.
The lawsuit is the largest civil rights action against a US firm
A federal appeals court upheld a 2004 ruling giving the lawsuit class action status, sanctioning claims from up to 1.5 million current and former staff.
Should it lose the case, the world's largest retailer could have to pay damages worth billions of dollars.
Wal-Mart has said it did not have a policy discriminating against women.
The world's largest retailer said it would appeal against the verdict.
The original lawsuit was filed in 2001 by six women who either worked for Wal-Mart or had done so in the past.
A lawyer representing the women said they had "been waiting years for this decision".
In a split two-to-one verdict, the San Francisco court ruled that the country's largest class action lawsuit against a private employer could proceed.
Judge Martin Jenkins said sufficient evidence existed of discriminatory practices dating back to 1998 to support the case going to trial.
"Factual evidence, statistical evidence and anecdotal evidence present significant proof of a corporate policy of discrimination and support plaintiff's contention that female employees nationwide were subjected to a common pattern and practice of discrimination," he said.
But in his dissenting opinion, Judge Andrew Kleinfeld said the only evidence of discrimination provided was the fact that the number of female managers at Wal-Mart stores was disproportionately lower than the total number of female staff.
"This case poses a considerable risk of enriching undeserving class members and counsel, but depriving thousands of women actually injured by sex discrimination of their just due," he argued.
Wal-Mart employs 1.3 million US workers, about 65% of them female
Whatever the outcome of Wal-Mart's appeal, the case - first heard in 2003 - is unlikely to come to trial for some time.
The lawsuit only applies to women employed by Wal-Mart since 26 December 1998.
At any future trial, the plaintiffs will need to establish that Wal-Mart had a company-wide policy of paying female staff less than men and that workers had no right to argue their individual cases.
Lawyer Brad Seligman, who is representing the women who brought the case, said the merits of the case had now been recognised twice.
"We fully expect Wal-Mart to keep appealing but we are very confident now that two courts have upheld this certification," he said.
Wal-Mart has argued that granting the lawsuit class action status is inappropriate because its 3,400 stores operate as individual businesses and that issues of pay and promotion are decided locally.
It said workers who believed they were victims of discrimination could sue individual stores.
Criticised in the past for poor employment practices, something which it has always denied, Wal-Mart has launched a host of diversity and environmental initiatives in recent years.
But last year the retailer was ordered to pay at least $78m in compensation to workers after a court found it had broken the law by not paying staff for working during breaks.