We now know Diane Keaton's cause of death. According to a statement from the star's family that was first published on Wednesday night by People, the legendary actress who passed away at 79 years old just a few days ago was felled by pneumonia. Related: Al Pacino Regrets Never Marrying The 'Love Of His Life' Diane Keaton... In a message the mourning loved ones shared with the mag just a few hours ago, the Oscar-winning actress' family confirmed that she died of pneumonia back on October 11. Wow.