The study was designed to define some of the elements of cognitive impairments evident in unmedicated Parkinson's disease (PD) patients. Unlike patients with progressive dementias such as Alzheimer's disease, untreated, mildly to moderately affected PD patients are efficient at accessing previously acquired knowledge. They also can learn and remember information that can be processed "automatically," using operations requiring little cognitive capacity. However, PD patients demonstrate relatively specific cognitive impairments for effort-demanding processes. These findings are clinically useful, but also help define the boundaries of psychobiologically distinct cognitive (memory-learning) processes.