The blaze erupted around 9:15 p.m. Wednesday in the 200 block of Greenfield Avenue.
Firefighters arrived on the scene to find heavy flames coming from the second floor of the home. The fire was extinguished in about 30 minutes.
Lower Merion Twp. fire officials say a woman in her 90s and her son, who is in his 60s, died in the blaze. They were both found in a back bedroom.
The man was in the doorway and his mother was found still in her bed.
"It's a tough day for our guys and for the neighbors," said James McCoy, the chief of the Lower Merion Fire Department. "We haven't had two fatalities in our township for as long as we can all remember."
A 29-year-old woman who was staying at the house is being treated at the hospital for non-life-threatening injuries.
The cause of the fire is under investigation, but it's believed to be accidental. The fire was mostly contained to the back bedroom.
Investigators said that smoke detectors were in the home, but they didn't work.
"We found smoke detectors with no batteries," said McCoy. "This house had about five or six smoke detectors, none with batteries that worked."
The names of the victims have not been released.