You Cannot Be Serious (ISBN 0-425-19008-0 in the US, Serious ISBN 0-7515-3454-4 in the UK) is a book written by professional tennis player John McEnroe, with James Kaplan.

The book made it to the top of the New York Times bestseller list.

Putnam's Sons in 2002, with the paperback being released a year later by The Berkley Publishing Group in the U.S., and by Time Warner in the UK.

from his match against Tom Gullikson at the 1981 Wimbledon Championships.

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