Pratchett, 59, whose Discworld books have sold 55 million copies worldwide and have been translated into some 27 languages, pledged to keep working and reassured fans there was "time for at least a few more books yet".
"I have been diagnosed with a very rare form of early onset Alzheimer's," he said in a statement entitled "an embuggerance" posted on the website of Discworld illustrator Paul Kidby.
"We are taking it fairly philosophically ... and possibly with a mild optimism."
Keep things cheerful
He said he would prefer it if fans "kept things cheerful" before
adding a typically witty postscript to underline his point.
"I would just like to draw attention to everyone reading the above that this should be interpreted as 'I am not dead.'" he wrote.
"I will, of course, be dead at some future point, as will everybody else.
"For me, this maybe further off than you think - it's too soon to tell.
"I know it's a very human thing to say 'Is there anything I can do', but in this case I would only entertain offers from very high-end experts in brain chemistry."
The 36 Discworld books are set in a flat, parallel universe balanced on the back of four elephants which themselves stand on the shell of a giant turtle.
Pratchett was awarded the OBE (Officer of the Order of the British Empire) in 1998 for services for literature and won the Carnegie Medal for children's literature in 2001. – (Sapa/AFP)
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Alzheimer's Centre
December 2007