Mental symptoms increased in frequency among 100 patients with parkinsonism treated with levodopa. Dementia was found in about one-third of patients throughout the 6-year treatment period. Thirteen patients became demented during the study, and dementia worsened severely in seven others. Agitated confusion became increasingly frequent and was observed in 60 percent of patients taking levodopa for 6 years. Withdrawal from levodopa decreased agitation, but not dementia. Ten patients received L-tryptophan along with levodopa, but no change in mentation was observed. In view of previous studies of mentation in Parkinson's disease and reports of widespread neuronal changes in the brain of autopsied patients with parkinsonism, our results suggest that the high incidence of dementia in patients with Parkinson's disease who take levodopa reflects prolongation of the course of the illness rather than a direct effect of the medication.