Originally recorded by David and Jonathan, and then Gene Pitney in 1967, the latter's version of the song reached No. 5 on the UK Singles Chart in December 1967, but failed to chart in the United States. The song was subsequently covered by a number of other acts including Cilla Black, Allison Durbin (dubbed "Australia's Queen of Pop" in the 1960s) on her 1968 album I Have Loved Me a Man, Terry Reid and Nick Cave and the Bad Seeds on their 1986 album of covers Kicking Against the Pricks. It was sung in French by Herbert Léonard in 1968 with the title "Quelque chose tient mon cœur" and in Italian by Pitney himself with the title "Uomo, non sai" ("Man, you don't know").

It achieved its greatest success in 1988 when it was covered by Marc Almond. Originally recorded by Almond alone for his 1988 album The Stars We Are, the single version reunited it with its original singer Gene Pitney; their version became a number one single in the UK for four weeks in January 1989. The music video for this version was filmed on-location in Las Vegas.

The song was covered by Vicky Leandros in 1968, along with a Greek version the same year titled "To Mistiko Sou" (Your secret) on her Greek album Vicky (To Mistiko Sou) which became a hit. The same year in French by Herbert Léonard "Quelque chose en moi tient mon cœur". Following Marc Almond's success across Europe, in 1989 Leandros released a 12-inch remix of "To Mistiko Sou" in Greece which topped the Greek charts. Since its release, both the English and Greek versions of the song have appeared on various Leandros compilations worldwide. In 2010, Greek singer Maro Lytra covered Leandros' Greek version of the song produced by Dimitris Kontopoulos, and released it as the second single off her upcoming album. In 1986, the song was covered by Nick Cave & The Bad Seeds on the album Kicking Against the Pricks.

...
Wikipedia