The cognitive performance of a group of 82 newly diagnosed patients with Parkinson's disease who had never been treated was reassessed approximately 4 mths after randomization to one of three monotherapies (levodopa, bromocriptine or anticholinergic drugs). Dopaminergic and anticholinergic treatments both led to improvement in motor control but their effects upon cognitive performance dissociated. Anticholinergic drugs produced impairment in processes underlying the immediate registration of information whilst dopaminergic therapy produced improvement on a task dependent on working memory and cognitive sequencing. Other cognitive measures showed no change on treatment. The deficits that were affected by cholinergic and dopaminergic modulation are those that were most compromised in the early, untreated state in Parkinson's disease. The data support the notion that cognitive impairment in Parkinson's disease is multifactorial in origin: short-term memory processes are served by both dopaminergic and cholinergic subcortico-frontal systems but much of the cognitive impairment of Parkinson's disease is independent of this subcortical neurochemical pathology and may be due to early neuronal dysfunction within the cerebral cortex.