After nearly a decade of legal wrangling, the U.S. Court of Appeals for the 9th Circuit on Monday certified the largest civil class action in U.S. history, giving ultimately as many as 1.5 million women the green light to proceed with sex discrimination claims against Wal-Mart Stores Inc.

The lawsuit — originally filed in 2001 by California Wal-Mart greeter Betty Dukes and five female colleagues — alleges that women employees are paid less and promoted less often than their male counterparts at Wal-Mart stores across the country.

In its 137-page decision on April 26, the 9th Circuit, sitting en banc, ruled 6-5 in favor of certification, rejecting Wal-Mart’s argument that the case was too large to be a class. “[M]ere size does not render a case unmanageable,” Judge Michael Daly Hawkins wrote for the majority.

Judge Susan Graber concurred, writing, “If the employer had 500 female employees, I doubt that any of my colleagues would question the certification of such a class. Certification does not become an abuse of discretion merely because the class has 500,000 members.”

The court did agree to reduce the size of the original class by up to two-thirds — from 1.5 million to 500,000 — by holding that those not employed by Wal-Mart in 2001 lacked standing in this action. But the court said those employees might still be eligible for back pay under a separate class action.

The dissenting judges were harshly critical of the ruling. “Never before has such a low bar been set for certifying such a gargantuan class,” wrote Judge Sandra Ikuta.

Chief Judge Alex Kozinski said in his dissent, “Maybe there’d be no difference between 500 employees and 500,000 employees if they all had similar jobs, worked at the same half-billion square foot store and were supervised by the same managers. But the half-million members of the majority’s approved class held a multitude of jobs, at different levels of Wal-Mart’s hierarchy, for variable lengths of time, in 3,400 stores, sprinkled across 50 states, with a kaleidoscope of supervisors….They have little in common but their sex and this lawsuit.”

In statements issued shortly after the decision, Wal-Mart’s general counsel and its lead appellate lawyer both suggested the retailer is considering an appeal to the U.S. Supreme Court.

For the plaintiffs, that could mean yet another delay as the defense has fought certification for nearly a decade.

“We’ve been on pins and needles for a very long time. The [9th Circuit] decision is just very exciting,” said Jocelyn Larkin of the Berkeley, Calif.-based Impact Fund and co-lead counsel for the plaintiffs, who noted it’s been an uphill battle.

“We appreciate that when you try to take on the largest employer in the U.S., that it ain’t gonna be easy. Well, it certainly has not been easy. It’s been a long wait, and we’re not nearly done,” Larkin said. She said she anticipates the case will go to trial.

Wal-Mart has long denied engaging in systemic discrimination. In response to the court’s ruling, the retailer issued a statement defending its pay and promotion practices. “We…are considering our options, including seeking review from the Supreme Court,” Jeff Gearhart, executive vice president and general counsel for Wal-Mart, said in the statement. “It is important to remember the court did not address the merits of this case.”

Gearhart added, “We do not believe the claims alleged by the six individuals who brought this suit are representative of the experiences of our female associates. Walmart is an excellent place for women to work and fosters female leadership among our associates and in the larger business world.”

Wal-Mart’s lead lawyer on appeal, Theodore Boutros Jr. of Gibson, Dunn & Crutcher in Los Angeles, also criticized the 9th Circuit’s ruling, saying in a statement that the decision violates “both due process and federal class action rules, contradicting numerous decisions of other federal appellate courts and the Supreme Court itself.” He added, “These are exceedingly important issues that reach far beyond this particular case and warrant Supreme Court review.”

Wal-Mart has challenged class certification in the Dukes case several times, never successfully. A federal judge ruled in favor of class certification in 2004. In 2007, a three-judge panel of the 9th Circuit affirmed the ruling, but later withdrew its initial opinion and then issued another ruling upholding certification. An en banc hearing was granted in February 2009, with the full court ruling against Wal-Mart on Monday.

The plaintiffs are also represented by co-lead counsel Joseph Sellers of Cohen Milstein Sellers & Toll in Washington as well as Equal Rights Advocates of San Francisco, the Public Justice Center in Baltimore, Davis, Cowell & Bowe in San Francisco, and Tinkler Law Firm and Merit Bennett in Santa Fe, N.M.

Tresa Baldas can be contacted at tbaldas@alm.com.