Sam's Club, which is facing fierce competition from rival Costco, has been trying to rev up its business with trendier home and fashion assortments and offering more incentives in its membership program.
"Stronger sales in the U.S. businesses would've helped our profit performance in the quarter," said McMillon in a transcript of a prerecorded call Thursday. "We can get better operationally ... and we will."
Net sales increased 2.7 percent at the Wal-Mart U.S. discount business, while rising 3.1 percent in its international business and 2.3 percent at Sam's Clubs.
Wal-Mart said it now expects earnings per share for the year to be in the range of $4.90 to $5.15 per share. That's down from its previous guidance of $5.10 to $5.45 per share.