In a cross-sectional study performed in 1980 on 70 consecutive Parkinson's disease (PD) outpatients, we investigated the factors associated with dementia, especially anticholinergic drugs. All cases fulfilled three major clinical criteria of PD, and underwent extensive clinical and laboratory examinations, including brain CT and neuropsychological assessment. Cases with mental deterioration at the onset of the illness or confusional status were excluded. In 15 patients the diagnosis of dementia was made according to DSM-III criteria; 15 other non-demented patients scoring 4 on the Reisberg's Global Deterioration Scale were labelled as "mentally deteriorated", and the remaining 40 cases were considered cognitively normal. In a logistic multiple regression analysis only age, female sex and time of anticholinergic intake were significantly associated with dementia. We conclude that anticholinergic drugs must be avoided in PD patients with some cognitive decline.