The Collaborative International Dictionary of English v.0.48:

Monomaniac \Mon`oma"ni*ac\, Monomaniacal \Mon`oma"ni*a*cal\, a.
   [Cf. F. monomaniaque.]
   Affected with monomania, or partial derangement of intellect;
   caused by, or resulting from, monomania; as, a monomaniacal
   delusion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

monomaniacal
    adj 1: obsessed with a single subject or idea