May same-store sales rose 1 percent, while analysts had expected a 2.2 percent rise, according to an average forecast compiled by Thomson Reuters.
Pharmacy same-store sales rose just 1.5 percent, feeling the impact of lower-priced generic drug introductions and one fewer weekday for consumers to fill prescriptions. Analysts, on average, had expected a 3.5 percent rise.
Same-store sales of general merchandise rose 0.2 percent, topping analysts' expectations for a 0.2 percent decline.
Total May sales rose 6.1 percent to $5.37 billion.
Walgreen said it had 6,857 drugstores as of May 31, 605 more than a year before.
(Reporting by Jessica Wohl, editing by Gerald E. McCormick)