WordNet (r) 3.0 (2006):

mental illness
    n 1: any disease of the mind; the psychological state of someone
         who has emotional or behavioral problems serious enough to
         require psychiatric intervention [syn: mental illness,
         mental disease, psychopathy] [ant: mental health]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "mental illness":
   aberration, abnormality, alienation, brain damage, brain disease,
   brainsickness, clouded mind, crack-up, craziness, daftness,
   dementedness, dementia, derangement, disorientation, distraction,
   emotional disorder, emotional instability, folie,
   functional nervous disorder, furor, insaneness, insanity,
   irrationality, loss of mind, loss of reason, lunacy, madness,
   maladjustment, mania, manic-depressive psychosis, melancholia,
   mental deficiency, mental derangement, mental disease,
   mental disorder, mental disturbance, mental instability,
   mental sickness, mind overthrown, mindsickness, nervous breakdown,
   nervous disorder, neurosis, oddness, paranoia,
   personality disorder, pixilation, possession, problems in living,
   psychosis, queerness, rabidness, reaction, reasonlessness,
   schizophrenia, senselessness, shattered mind, sick mind, sickness,
   social maladjustment, strangeness, unbalance, unbalanced mind,
   unsaneness, unsound mind, unsoundness, unsoundness of mind,
   witlessness