BACKGROUND: Age-related declines in intellectual functioning have been linked to slower processing of information. However, any slowness with advancing age could simply reflect slower movement rather than impaired cognition. To assess any age-related decline in cognitive speed, we used an accuracy-based task that does not require a speeded motor response and that measures the time required to acquire information (inspection time). To identify possible biological mechanisms of cognitive slowing, this task was also applied to patients with Parkinson's disease, a basal ganglia disorder that reportedly causes bradyphrenia (slower thought processes). METHODS: In one experiment, 16 young (mean age 22.4 years) and 16 older adults (mean age 71.6 years) matched for intelligence and education completed an inspection time task. The task required judgments as to order of onset of two lights, where the interval between onsets ranged from 20-250 msec. A second experiment compared 16 patients diagnosed with idiopathic Parkinson's disease and 16 age-matched controls upon the same task. RESULTS: Older adults demonstrated significant cognitive slowing compared to younger adults. Medicated nondemented Parkinsonian patients were not impaired on this task compared to age-matched controls. CONCLUSIONS: Clinical and empirical impressions of bradyphrenia in Parkinson's disease may instead reflect advancing age or slower movement, because the effects of age may be greater in some cases than the effects of basal ganglia disease once motor dysfunction has been allowed for.