Woody Harrelson wants Matthew McConaughey to take a DNA test to see if they're bothers.
The Dallas Buyers Club star, 53, said it was a possibility after revealing his mum "knew" Woody's dad when she was separated from her husband.
The White Men Can't Jump star, 61, has now suggested his pal find out if they are related for real.
Speaking on the Late Show with Stephen Colbert, Woody said: "Well, I'll just say there is some veracity to that thought because we talked to Ma' Mac, Matthew's legitimate mother, and she let us know one time… I mean, this is crazy.
"We were in Greece watching the US team win the World Cup and I don't know, I mentioned something about regrets."
The True Detective star continued: "And I said, 'You know, it's odd that my father has no regrets.' And I have known Ma' Mac a long time, and she goes, 'I knew… your father.' And it was the ellipses I found a little troubling, or interesting. 'I knew your father.'"
Describing Ma' Mac as "full of innuendo", Woody claimed Matthew was conceived the same time as their parents were reportedly acquainted.
"The year of his birth, nine months before, she was on a sabbatical from her relationship with his supposed father, Jim," he stated.
Despite being able to confirm their relationship with a DNA test, Woody conceded Matthew wasn't keen on getting one, explaining his friend felt like he could be losing a father.
He shared: "We want to go for a test, but for him, it's a much more big deal. I mean he feels like he is losing a father. But I'm like, no, you're gaining a different father and a brother."
The bombshell from Matthew's mum led to him investigating his family history.
He found out Woody's dad was on furlough at the same time his own parents were going through their second divorce.
“Look, it’s a little easier for Woody to say, ‘Come on, let’s do [DNA tests],’ because what’s the skin in it for him?” he told Kelly Ripa’s Let’s Talk Off Camera podcast.
Matthew added: "It’s a little harder for me because he’s asking me to take a chance to go, ‘Wait a minute, you’re trying to tell me my dad may not be my dad after 53 years of believing that?’ I got a little more skin in the game.”
Matthew and Woody are set to reunite on screen again in a new Apple TV+ comedy where they star as fictionalised versions of themselves as they blend their families under the same roof on a Texas ranch.
It is unclear if the real-life speculation factors into the 10-part series, Brother From Another Mother.
