The justices will decide whether the women can sue the nation's largest retailer for sexual discrimination.
If they win, it would cost Wal-Mart billions.
The 10-year-old lawsuit claims that Wal-Mart favors men over women in pay and promotions and of fostering an environment where sexism is pervasive.
"Before I was promoted, when I was asking what I need to do, I was told I needed to blow the cobwebs off my make-up and to doll up," said Christine Kwapnoski.
The chants by demonstrators outside the Supreme Court on Tuesday were hard to ignore.
The Supreme Court must now decide not whether thousands of Wal-Mart's female employees were discriminated against, but whether they should be allowed to bring the biggest class action lawsuit ever against the world's largest employer.
The attorney for the plaintiffs said class action is necessary because the average worker could not afford to take her case to court.
Some shoppers at Southern California stores said they will continue to spend their money at Wal-Mart no matter how the high court rules.
"It wouldn't keep me from coming here completely," said David Vos of Santa Clarita.
The Associated Press contributed to this report.