(CNN) - A flash mob showed up to give their support to a teenager in St. Louis fighting leukemia.
Seth Lange, 16, received the visit ahead of bone marrow transplant surgery.
They sang for Seth in an effort to get him to flash a smile.
"The thing Seth is known for is that smile, and that smile is contagious," Stara Lockard said.
At 8:30 Monday, the flash mob took over Cardinal Glennon. Seth could hear the singing from his hospital bed.
When he walked to his window, he realized it was all for him.
"God bless you all," Seth said. "Thank you very much."
Seth has been fighting leukemia for two years. On Tuesday he had the bone marrow transplant.
"It`s been an emotional roller coaster from day one," Seth's mom, Stacy Vogt, said. "But just to see the support out here tonight. It`s just amazing."
The flash mob was organized through Facebook by Shawn Kohrs.
"I thought big equals people, and how do we get people to see him? It's not going to happen. He's in isolation," Shawn said. "So I thought, 'Let's do a flash mob.'"
More than 200 people attended.