The Collaborative International Dictionary of English v.0.48:

Ambitiousness \Am*bi"tious*ness\ ([a^]m*b[i^]sh"[u^]s*n[e^]s),
   n.
   The quality of being ambitious; ambition; pretentiousness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ambitiousness
    n 1: a strong drive for success [syn: ambition,
         ambitiousness]