Will Hart, 14, wrote, "Hi Mom, God bless you" on top of a Chicago medical center's parking garage.
His mother, 48-year-old Sharon Hart, was recently diagnosed with leukemia and has been at the hospital since last week for chemotherapy.
Will got some help from his dad and when they were finished, he called her up and told her to look outside.
"We drove up there. We were going to go see the city. I just kind of thought, you know what, it would be great to have my mom to see this message. And so I started from there," said Will.
"I was amazed and proud at the same time, and of course full of love, to think he would greet me like that was just wonderful. It lifted me up entirely," said Sharon.
Nurses at the hospital say the message touched all of their hearts, especially the other cancer patients.