A fire broke out at the Parkland View Apartments along Cetronia Road in Upper Macungie Township around 11:30 a.m. Sunday.
Emergency crews quickly arrived on the scene, and the fire is now extinguished.
There was extensive damage to the building.
According to Upper Macungie Township Fire Commissioner Grant Grim, wind was a major factor in the spread of the fire.
He said the sprinkler systems were working, but once the fire got under the roof and the wind got a hold of it, all was lost.
He said more than 20 people were displaced but no injuries have been reported.
The American Red Cross says it has assisted 23 people (20 adults, 3 children) and will continue to assess their needs.
The cause of the fire is still under investigation.