Lifelong pals Alan Robinson and Walter MacFarlane have been best mates for 60 years, ever since they played football together at prep school in Hawaii.
So the pair have been left "overwhelmed" after discovering that they are not just friends, but brothers too.
The men learned of their sibling bond after separately taking DNA tests to trace details about their ancestry.
Mr MacFarlane never knew his father, while Mr Robinson was adopted, Honolulu news station KHON-TV reported.
After using DNA-matching websites following unsuccessful searches on social media and other sites, Mr MacFarlane found a top match – someone with identical X chromosomes – with the username Robi737.
Mr Robinson, who flew Boeing 737 planes for Aloha Airlines, had used the same website to find answers about his ancestry, it transpired.
After realising the connection, the pair learned they have the same birth mother.
They said it had been an "overwhelming experience" as they revealed the news to friends and family.
"This is the best Christmas present I could ever imagine having," Mr Robinson said.
"I had a younger brother that I lost when he was 19, so I never had nieces or nephews. I thought I'll never know my birth mother, I'll never have any nieces or nephews."
Both men said it had never crossed their minds that they might be related.
"But there was times when I did think, you know, I look like Robbie, a little bit," Mr MacFarlane said.
The pair are now planning to enjoy retirement together as brothers.
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