Many states use a formula consisting of the relative in-state sales, payroll, and property to apportion income of a multistate taxpayer. States can change the apportionment formula by modifying the weight assigned to these factors. By using a throw-out rule, states may adjust the sales factor by removing from the denominator gross receipts that are not sourced to any state, thus mathematically increasing the sales fraction and the amount of income apportioned to the state. In the last few years, the throw-out rule has become significantly relevant in state and local taxation with its adoption in Alabama, Maine, and Washington and its repeal in New Jersey. Prior litigation and the recently decided Whirlpool case in New Jersey demonstrate that not all applications of throw-out rule, which often result in large assessments to the corporate taxpayers, are legal. This Article reports on an in-depth investigation of the throw-out rule used by the taxing jurisdictions to alter the apportionment formula. It proposes practical litigation strategies of challenging the application of existing and future throw-out rules as they may apply to the apportionment of the corporate taxpayers’ multistate income.