The 200-person party was under way at a short-term rental property at Madison Avenue and Suismon Street when nearly 50 shots were fired around 12:30 p.m., the Associated Press reports.
Some people purportedly broke bones jumping out of windows while trying to flee.
"You have alcohol, you have underage people here and you have guns," Pittsburgh police commander John Fisher told WPXI-TV.
"That's a deadly combination at any type of an event, and the end result is it’s a tragedy."
Click here to follow Daily Voice Lower Merion-Narberth and receive free news updates.