The Collaborative International Dictionary of English v.0.48:

Truthful \Truth"ful\, a.
   Full of truth; veracious; reliable. -- Truth"ful*ly, adv.
   -- Truth"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

truthfully
    adv 1: with truth; "I told him truthfully that I had just
           returned from my vacation"; "he answered the question as
           truthfully as he could" [ant: mendaciously,
           untruthfully]