In treating PD, deep brain stimulation (DBS) has shown great promise in a series of uncontrolled studies. To estimate the incremental cost effectiveness of DBS compared with the best medical management in late-stage PD. The authors constructed a decision model to determine the lifetime incremental cost effectiveness between two options in patients with PD aged 50 years and older: 1) best medical management and 2) DBS. As the long-term efficacy of DBS (>3 years) is not known, key assumptions regarding the procedure's long-term durability were made. Costs were in US 2000 dollars, and quality-adjusted life year (QALY) was the effectiveness measure. Base assumptions were that quality of life (QOL) in patients with late-stage PD is 0.55 (0-to-1 scale, 1 is perfect health) and that DBS benefits are constant for 4 years, eroding gradually over the next 5 years until at parity with those produced by best medical management. Incremental cost-effectiveness and sensitivity analyses were performed. Under base-case assumptions, DBS provides an additional 0.72 QALY at an additional cost of $35,000 compared with best medical management that results in an incremental cost-effectiveness ratio (C/E) of $49,000. QOL increases of between 18 and 30% resulted in questionable cost effectiveness. QOL increases of between 6 and 18% resulted in incremental C/E ratios not usually considered cost effective (>100,000/QALY). The results suggest that DBS may be cost effective in treating PD if QOL improves 18% or more compared with those receiving best medical management. This underscores the need for randomized, controlled, prospective DBS experiments including QOL and economic components.