The attack happened Thursday morning at a high rise near Wabash and Hubbard.
A resident said after the intruder got into the building, he went to the mailroom and damaged packages. Then he somehow got into the fitness center, where he stabbed a resident who was exercising there.
The victim was rushed to the hospital with serious injuries. Chicago police arrested his alleged attacker.
In a letter to residents, the building's property manager said they are "reviewing all security conditions of the building."
Some residents said they feel very unnerved by how the alleged attacker was able to get into the building and continue on to the fitness room. Police have not identified the attacker or announced charges.