Sharon Hart, 48, who is receiving chemotherapy and other treatments for acute myeloid leukemia at Rush University Medical Center, said her son, Will Hart, 14, called her Saturday and told her to look out the window of her 12th-floor hospital room, the Chicago Sun-Times reported Wednesday.
Hart looked out the window and discovered Will, with the help of his father and uncle, had stamped out the words "HI MOM" in the snow on the roof of the adjacent parking garage.
"At that moment, I was feeling the most pure form of joy you could imagine," Hart said.
"I proceeded to blow him kisses and wave at him. ... Something as simple as a message in snow made me so happy, and I actually started crying a bit," she said.
Hart said she received another call a little while later to look out the window and read an addition to the message.
"GOD BLESS U," the new letters read.
"They wanted to write 'GOD BLESS U ALL,' but ran out of room," she said. "He also was thinking of everyone else in the hospital, to feel better and be lifted by this."
Jennifer Cranston, a registered nurse at the hospital, said news of the message spread quickly through the building.
"I think it just kind of warms the heart to see a child try to support their parent in such a lovely way," Cranston said.