Neighbors say they heard the explosion near the shuttered Edgewater Medical Center in the 5700-block of North Ashland.
Fire officials say two 15-year-old boys were playing on the roof of the hospital.
One was shocked by a transformer explosion, and is in critical condition.
The other was hurt by falling debris. He's in serious to critical condition.
Some in the neighborhood say the building needs to be more secured.
"Nobody in the neighborhood is surprised that something like that could conceivably happen....I could show you the guard shack -- it's not staffed," said Joseph Drantz, West Edgewater Area Residents Association.
The neighborhood association says a foreign bank now owns the hospital. It's been vacant since it closed in 2001.