Willis' family said he was recently "diagnosed with aphasia, which is impacting his cognitive abilities."
Bruce Willis' decision to end his acting career of more than four decades after a recent aphasia diagnosis has put a spotlight on the somewhat rare disorder.
Aphasia describes a neurological condition that affects a person’s ability to communicate verbally or through writing.
It affects an estimated 2 million people in the U.S., and nearly 180,000 acquire it every year, according to the National Aphasia Association.
Willis' daughter Rumer Willis posted Wednesday on Instagram that the condition has affected her father's "cognitive abilities."
Rumer Willis' mother, the actor Demi Moore, also shared the statement.
The most common cause of aphasia is brain damage from a stroke.
Willis, an Emmy-winning actor and the star of hit movies like "Die Hard" and "The Sixth Sense," turned 67 this month.