The employment discrimination lawsuit against Wal-Mart, which the Supreme Court heard last week, is the largest in American history. If the court rejects this suit, it will send a chilling message that some companies are too big to be held accountable.

It began in 1999 after Stephanie Odle was fired when she complained of sex discrimination. As Ms. Odle recounted in sworn testimony, as an assistant manager she discovered that a male employee with the same title and less experience was making $10,000 a year more than her.

She complained to her boss, who defended the disparity by saying the male had a family to support. When she replied that she was having a baby that she needed to support, the supervisor made her provide a personal budget and then gave her a raise closing just one-fifth the gap.

The plaintiffs who have brought a class action on behalf of 1.5 million current and former female Wal-Mart employees allege that they, too, faced discrimination in pay and promotion. If Wal-Mart loses, it could owe more than $1 billion in back pay.