Background: Most prognostic studies on Parkinson disease have been hospital based or have applied register-based casefinding methods. Potential under-representation of mild cases may have given biased results. Abstract:Objective: To evaluate whether Parkinson disease is associated with an increased risk of dementia and death. Design: Population-based cohort study. Parkinson disease and dementia were assessed through in-per-son examination at baseline (1990-1993) and 2 follow-up visits (1993-1994 and 1997-1999). Computerized linkage to medical and municipality records provided additional information on disease outcomes and mortality. Setting: General population. Participants: A total of 6969 participants, including 99 prevalent and 67 incident cases of Parkinson disease. Main Outcome Measures: Incident dementia and death. Adjusted hazard ratios were calculated through Cox proportional hazards regression analysis. Results: Patients with Parkinson disease had an increased risk of dementia (hazard ratio, 2.8; 95%confidence interval, 1.8-4.4), which was especially pronounced in participants carrying at least 1 apolipoprotein E gene (APOE) 2 allele (13.5; 4.5-40.6). Parkinson disease was associated with an increased mortality risk (1.8; 1.5-2.3). The association consistently diminished when analyses were sequentially restricted to patients with shorter disease duration and after adjustment for the occurrence of dementia. Conclusions: Especially patients with Parkinson disease who carry an APOE 2 allele have an increased risk of developing dementia. Increased mortality risk in Parkinson disease is dependent on disease duration and is only modest in the absence of dementia. Background: Most prognostic studies on Parkinson disease have been hospital based or have applied register-based case binding methods. Potential under-representation of mild cases may have given biased results. Abstract: Objective: To evaluate whether Parkinson disease is associated with an increased risk of dementia and death. Design: Population-based cohort study. Parkinson disease and dementia were assessed through in-per-son examination at baseline (1990-1993) and 2 follow-up visits (1993-1994 and 1997-1999) to medical and municipality records provided additional information on disease outcomes and mortality. Settings: General population. Participants: A total of 6969 participants, including 99 prevalent and 67 incident cases of Parkinson disease. Main Outcome Measures: Incident dementia and death. Adjusted hazard ratios were calculated through Cox proportional hazards regression analysis. Results: Patients with Parkinson disease had an increased risk of dementia (hazard ratio, 2.8; 95% confidence interval, 1.8-4.4), which was especially pronounced in participants carrying at least 1 apolipoprotein E gene (APOE) 2 allele (13.5; 4.5-40.6) The association consistently diminished when analyzes were were restricted to patients with shorter disease duration and after adjustment for the occurrence of dementia. Conclusions: Especially patients with Parkinson disease who carry an APOE 2 allele have an increased risk of developing dementia. Increased mortality risk in Parkinson disease is dependent on disease duration and is only modest in the absence of dementia.