This controlled prospective study examined the evolution and predictors of cognitive decline in Parkinson's disease (PD). Consecutive patients diagnosed at baseline with PD (n = 89), established PD (EPD) patients (n = 52) with a mean disease duration of 6.5 years, and healthy control subjects (n = 64) underwent extensive neuropsychological assessment twice, approximately 3 years apart. A standardized regression-based method, normative data, and multivariate normative comparisons were used to assess the cognitive course of PD. Cognitive performance of newly diagnosed patients decreased significantly over time, particularly on measures of psychomotor speed and attention and to a lesser extent on tests of memory, visuospatial skills, and executive functions. About 50% of the patients showed cognitive decline and 9% developed dementia. Similar results were observed in EPD patients. None of the baseline features predicted cognitive change in newly diagnosed patients, whereas age at disease onset and axial impairment (postural and gait disorders) contributed to decline in established patients. We conclude that within few years after diagnosis, PD patients show faster rate of cognitive decline than matched healthy subjects, particularly in domains of attention and psychomotor speed. Selection bias probably led to underestimation of the true extent of cognitive decline in established patients.