A new biography of Dean Martin has revealed the late singer's links to the mafia, through files collected by the US Federal Bureau of Investigation (FBI).
Many of Martin's hits remain popular today
Martin was given help with his early singing career by mob bosses who owned saloons in Chicago, according to Michael Freedland's book Dean Martin: King Of The Road.
The singer, whose hits included That's Amore and Volare, subsequently repaid them by performing on demand when he became a major star.
"Unlike other people, he knew to say 'thankyou' afterwards," Freedland told BBC World Service's The Music Biz programme.
"If Sam Giancana, the real godfather of Chicago, decided he wanted to put on a show - and this is borne out by FBI files that I saw - he would say, 'get Dean Martin, I need him for 10 days.' And he would go.
"But he could have got himself into very serious trouble."
Many singers at the time were believed to have had connections to the mob.
Frank Sinatra, the most high-profile of the so-called "rat pack" of singing stars that also included Martin and Sammy Davis Jr, was long suspected of involvement - although he denied this up until his death in 1998.
But Freedland said that often, the FBI would turn up at Martin's concerts to observe who was there - as there was a guarantee of tracking down many with links to the mob.
Martin made his name alongside Jerry Lewis
He quoted Martin as saying that at one performance at Atlantic City, "if somebody threw a bomb into the building on that day, organised crime would be wiped out in one moment."
"That shows you - they were all there," Freedland said.
Freedland's book also details some of Martin's insecurities - in contrast to his popular image of laidback cool.
He said that in particular, he felt inferior in comparison to Jerry Lewis - his partner in a total of 14 films in the 1950s, including Sailor, Beware!, Pardners and Hollywood Or Bust.
Martin usually supplied the songs and romantic interest in these films - the straight man for comedian Lewis to bounce off.
"I think [Martin] felt inferior at the time when undoubtedly Jerry was a star,"
"It was a comedy partnership, so the funny one got the laughs."
He also quoted Martin as once telling him: "the greatest thing I ever did was to team up with Jerry Lewis - the second-greatest thing I ever did was to leave Jerry Lewis."