Medical terms can get quite long
pneumonoultramicroscopicsilicovolcanoconiosis is 45 letters.
As the largest known protein, Titin also has the longest IUPAC name. The full chemical name, which starts Methionyl and ends with isoleucine contains 189,819 letters and is sometimes stated to be the longest word in the english language. I
t can take over three hours to pronounce.However, lexicographers regard generic names of chemical compounds as verbal formulea rather than English words. - wikipedia
If you would like to see the full name of Titin go to
http://www.sarahmcculloch.com/lumina.../longest-word/
But I think there is a typo on line 1245.
The longest word to appear in literature is
Lopadotemachoselachogaleokranioleipsano drimhypotrimmatosilphioparaomelitokat akechymenokichlepikossyphophattoperist eralektryonoptekephalliokigklopeleiola goiosiraiobaphetraganopterygon which appeared in Aristophanes' Greek comedy Assemblywomen.