The Collaborative International Dictionary of English v.0.48:

Harmful \Harm"ful\ (h[aum]rm"f[.u]l), a.
   Full of harm; injurious; hurtful; mischievous. " Most harmful
   hazards." --Strype. --Harm"ful*ly, adv. -- Harm"ful*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

harmful
    adj 1: causing or capable of causing harm; "too much sun is
           harmful to the skin"; "harmful effects of smoking" [ant:
           harmless]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "harmful":
   adverse, antagonistic, bad, bad for, baleful, baneful, bitchy,
   conflicting, contaminated, contrary, corroding, corrosive,
   corrupting, corruptive, counter, counteractive, counterproductive,
   cussed, damaging, dangerous, deadly, deleterious, destructive,
   detrimental, difficult, disadvantageous, disserviceable,
   distressing, evil, foul, hard, hateful, hazardous, hostile,
   hurtful, ill, in opposition, inimical, iniquitous, injurious,
   insalubrious, insanitary, invidious, lethal, malefic, maleficent,
   malevolent, malicious, malign, malignant, mean, mischievous,
   miserable, morbific, nasty, noisome, not easy, noxious, ominous,
   opposed, opposing, opposite, ornery, pathogenic, peccant,
   pernicious, pestiferous, poisonous, polluted, prejudicial,
   rigorous, risky, scatheful, septic, sinister, stressful, tainted,
   toxic, troublesome, troublous, trying, unadvantageous, unfavorable,
   unhealthful, unhealthy, unhygienic, unprofitable, unrewarding,
   unsafe, unsanitary, untoward, unwholesome, useless, venenate,
   veneniferous, venenous, venomous, vicious, virulent, wicked,
   worthless, wretched