In this paper, a fuzzy operator for contrast enhancement of mammography images is presented. This intensity modification method is effective and fast in computing. In this approach, the selection of appropriate parameters for the required transformations is performed based on the specific image characteristics. Images are first transformed using a fuzzification function. We then apply an algorithm for intensity adaptation based on involutive fuzzy complements and measures of fuzziness. Regarding to the amount of ambiguity the proposed technique detects the suitable form of the modification in the set of involutive membership function. Finally, the defuzzification procedure is performed to transform the modified image back to the spatial domain. The experimental results show that this method can enhance the region of interest in mammography images that is useful for breast cancer diagnosis.