The extent to which concomitant Alzheimer's disease (AD) is etiologically related to the development of dementia in Parkinson's disease (PD) remains controversial. We explored the association of four risk factors associated with AD, including head injury, smoking, hypertension, and diabetes mellitus, with incident dementia in PD. A cohort of 180 nondemented PD patients from the Washington Heights community in northern Manhattan, New York, completed a risk factor questionnaire at baseline and was followed annually with neurological and neuropsychological evaluations. The association of baseline variables with incident dementia was analyzed by using Cox proportional hazards models. All analyses controlled for age at baseline, gender, years of education, duration of PD, and total Unified Parkinson's Disease Rating Scale (UPDRS) motor score at baseline. Of 180 patients (mean age, 71.0 ± 10.3 years), 52 (29%) became demented during a mean follow-up period of 3.6 ± 2.2 years. Head injury risk ratio ([RR] 0.9; 95% confidence interval [CI], 0.4–2.2; P = 0.9), hypertension (RR, 0.7; 95% CI, 0.4–1.4, P = 0.3), and diabetes mellitus (RR, 0.8; 95% CI, 0.3–2.3; P = 0.7) were not significantly associated with incident dementia in the Cox models. Patients who reported having ever smoked were at increased risk for the development of dementia compared with nonsmokers (RR, 2.0; 95% CI, 1.0–3.9; P = 0.05). Current smoking was significantly associated with incident dementia (RR, 4.5; 95% CI, 1.2–16.4; P = 0.02), whereas past smoking approached significance (RR, 1.9; 95% CI, 0.9–3.7; P = 0.07). Although an inverse association between smoking and PD has been reported in several studies, our study showed a positive association between smoking and dementia in the setting of PD. The association of smoking with incident dementia in PD deserves further study. © 2002 Movement Disorder Society.