OBJECTIVE: Budget impact analysis (BIA) quantifies the potential incremental costs of new health technologies for health care payers. However, technologic advances often come in waves, with multiple products in a therapeutic area entering the market simultaneously. To present a method for using dynamic baseline market shares to estimate the budget impact of a single product when there are multiple market entrants. METHOD: In a BIA, current market shares of all existing products are typically used as a static scenario to estimate baseline costs. Against this baseline, future product market shares are used to estimate new costs and the budget impact is calculated as the difference between the baseline and new scenarios. However, using current market shares as a static baseline is problematic when there are multiple market entrants as it includes the cost impact of all new products since all start at 0% share in the BIA. To estimate the budget impact of a single product among simultaneous multiple market entrants requires that future market share projections for all products minus the product of interest be included in a dynamic baseline. The BIA is then calculated as the difference between the dynamic baseline scenario without the new product of interest and the new scenario with the product of interest. The advantage of this method is that the BIA is more flexible and can estimate the budget impact of a new single product or combinations of products. The primary challenge is that it is more difficult to estimate dynamic baseline market shares which are not based exclusively on current market share data. Worked examples to illustrate the methods will be provided. CONCLUSION: A BIA with dynamic, instead of static, baseline market shares provides a method to estimate the budget impact for single new products when there are simultaneous multiple market entrants.