Much controversy and debate surrounds Big Boss' real name. When Metal Gear Solid 3 was released players safely assumed his real name was Jack, as The Boss and Major Zero called him by this name on several occasions.
However, early on in the game Big Boss is asked by Para-Medic what his real name is, to which he responds "John Doe" with Para-Medic replying "and they call you Jack for short?" Due to slightly tongue-in-cheek nature of the conversation most initially thought it was nothing more than a joke. However, at the end of Metal Gear Solid 3, Big Boss also tells Ocelot that his name is John, and in Japan, a Limited Edition release of MGS3 included a book called R which stated Big Boss' real name was indeed John Doe. The master Patriot AI, accessed via Big Boss' biometric data, is also named John Doe. However, some fans still feel the name was just a joke, and refuse to acknowledge John Doe as Big Boss' real name.
To further add to the confusion, the Metal Gear Solid 4 Database listed Big Boss' name as simply "John." However, both names can be applicable as the name "Jack" in English is the diminutive form of the name "John."