The Collaborative International Dictionary of English v.0.48:

Fanatical \Fa*nat"ic*al\, a.
   Characteristic of, or relating to, fanaticism; fanatic.
   -Fa*nat"ic*al*ly, adv. -- Fa*nat"ic*al*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fanatical
    adj 1: marked by excessive enthusiasm for and intense devotion
           to a cause or idea; "rabid isolationist" [syn: fanatic,
           fanatical, overzealous, rabid]

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "fanatical":
   agog, ardent, at fever pitch, authoritarian, balking, balky, bigot,
   bigoted, borne, bulldogged, bulletheaded, bullheaded, burning,
   case-hardened, closed, compulsive, constricted, cramped,
   creedbound, deaf, deaf to reason, delirious, distracted, dogged,
   dogmatic, enthusiastic, excessive, extravagant, extreme, extremist,
   fanatic, febrile, fervent, fervid, feverish, fiery, flaming,
   frantic, frenetic, frenzied, furious, haggard, hard-core,
   hardheaded, headstrong, heated, hectic, hidebound, hot, hotheaded,
   hysteric, illiberal, immoderate, infatuated, inflamed, inordinate,
   insane, insular, intolerant, irrational, little, little-minded,
   mad, madcap, maniacal, mean, mean-minded, mean-spirited,
   monomaniacal, mulish, narrow, narrow-hearted, narrow-minded,
   narrow-souled, narrow-spirited, nearsighted, obsessive, obstinate,
   opinionated, overanxious, overdesirous, overdevout, overeager,
   overenthusiastic, overreligious, overrighteous, overzealous,
   parochial, passionate, perfervid, persevering, pertinacious, petty,
   pigheaded, provincial, purblind, rabid, red-hot, restive,
   sanctimonious, scorching, self-willed, set, shortsighted, small,
   small-minded, stiff-necked, straitlaced, strong-willed,
   strongheaded, stubborn, stuffy, sulky, sullen, tenacious,
   totally committed, ultrareligious, ultrazealous, uncatholic,
   uncharitable, uncooperative, ungenerous, unliberal, unreasonable,
   unregenerate, white-hot, wild, wild-eyed, wild-looking, willful,
   zealotic, zealous