Ultraliberals, ultraorthodox, ultrareliable, unabbreviated, unaccompanied, unaccountable, unadulterated, unanticipated, unappreciated, uncertainties, uncompetitive, unconditional, uncontainable, uncooperative and undeliverable are 13 letter words. Additional 13 letter words include underachiever, underclassman, undercurrents, undergraduate, understanding, undistributed, unenforceable, unfamiliarity, unforgettable, unimpeachable and unintimidated.
uniform
udon
There is NO word in the English language whereby the letter q is not followed by the letter u.
urchins
turquiose
unclean :)
unkempt
· unassuming
Underground is a 11 letter word starting with U and ending with D.
unreal
Ural
Ah, the usual question.