WordNet (r) 3.0 (2006):

mental hospital
    n 1: a hospital for mentally incompetent or unbalanced person
         [syn: mental hospital, psychiatric hospital, mental
         institution, institution, mental home, insane
         asylum, asylum]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "mental hospital":
   VA hospital, asylum, base hospital, bedlam, bughouse, clinic,
   community hospital, convalescent home, convalescent hospital,
   evacuation hospital, field hospital, general hospital, home,
   hospital, infirmary, inpatient clinic, insane asylum,
   lunatic asylum, madhouse, maison de sante, mental home,
   mental institution, nursing home, nuthouse, osteopathic hospital,
   outpatient clinic, padded cell, policlinic, polyclinic,
   private hospital, proprietary hospital, psychiatric ward,
   psychopathic hospital, public hospital, rest home, sanatorium,
   sick bay, sickbed, sickroom, special hospital, station hospital,
   surgical hospital, teaching hospital, veterans hospital,
   voluntary hospital, ward, well-baby clinic