Attempts to correlate the amount of bilateral brain injury with behavioral changes can be complicated if the lesions are bilaterally unsymmetrical in volume. Further complications are introduced if unilateral lesions are ineffective in producing behavioral changes. An index, W%, is proposed which is based on the product of the volumes of the damage on the right and left sides. The index ranges from 0 to 100 such that conditions of no injury or unilateral injury produce an index of zero. In contrast, relatively large values of the index are generated by symmetrical lesions. Grossly unsymmetrical lesions, in which one side is nearly intact, produce very low index values. For lesions that are bilaterally equivalent in volume, the percentage of tissue destroyed on either side is equal to 10 square root of W%. Some properties of the proposed index are discussed.