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Functional dependency (needing help with basic ADLs) is an important outcome in Parkinson's disease (PD). "Death or dependency", as opposed to being alive and independent, is a useful dichotomous indicator of poor outcome. We aimed to systematically review the progression to dependency in PD and what factors predicted development of dependency. Comprehensive searches were performed to identify observational studies of dependency in PD with follow-up of at least 3 years. Other forms of parkinsonism and highly selected cohorts were excluded. Descriptive analysis of included studies was performed and outcomes over time were plotted by type of cohort (inception/non-inception). Independent prognostic factors were identified. There were insufficient data for meta-analysis. Of 15,154 unique references identified, 14 studies were included. Most studies were of low quality. There was heterogeneity in definitions of dependency and the measured risk of dependency at similar time-points. Risk of dependency in inception studies was about 10-25 % at 5 years and about 20-50 % at 10 years; and risk of "death or dependency" in the inception studies was about 15-40 % at 5 years and about 35-70 % at ten years. More bradykinesia and older age were associated with more dependency, but there was little evidence for other prognostic factors. Few high-quality data on dependency are available. Heterogeneity in study populations, methodology and outcome reporting made data synthesis difficult. Few prognostic factors have been identified. Further data from representative inception studies are necessary to better understand the progression of dependency in PD.