wiki上有统计：
First Doctor: apparently succumbed to old age, steadily growing weaker throughout The Tenth Planet and collapsing at the serial's end. Although the writer's intent was that this was due to the energy drain from the planet Mondas, this was not made clear in the transmitted story.
Second Doctor: a forced "change in appearance" and exile to Earth by the Time Lords in the closing moments of The War Games.
Third Doctor: radiation poisoning from the Great One's cave of crystals at the end of Planet of the Spiders.
Fourth Doctor: fell from the Pharos Project radio telescope in Logopolis.
Fifth Doctor: spectrox toxaemia, contracted near the start of The Caves of Androzani.
Sixth Doctor: suffered unspecified injuries when the Rani attacked the TARDIS and caused it to crash land at the start of Time and the Rani.
Seventh Doctor: died in San Francisco during exploratory heart surgery by a doctor unfamiliar with Time Lord physiology, after being hospitalised for non-life threatening gunshot wounds in the 1996 television movie.
Eighth Doctor: Not yet revealed.
Ninth Doctor: cellular degeneration caused by absorbing the energies of the space-time vortex from Rose, which she in turn had absorbed through the heart of the TARDIS in "The Parting of the Ways".
Tenth Doctor: Current incarnation.
出处：
http://en.wikipedia.org/wiki/Doctor_(Doctor_Who)
