This is the first report to my knowledge of neuroleptic-induced dementia in adults with mental retardation. Four adults showed full recovery after having significant cognitive deterioration that persisted until the offending medication was withdrawn. The ages at onset of dementia were 37, 37, 39, and 55 years; the duration of dementia ranged from .5 to 2.5 years. Three adults had Down syndrome and one had mental retardation due to an unknown cause. The neuroleptic-induced dementia occurred while the individuals were on a low dose of either loxapine, thioridazine, haloperidol, or pimozide.