Endometrial polyps are benign lesions frequently identified in women with infertility or abnormal uterine bleeding in the reproductive and postmenopausal phases We report the striking observation that the numbers of activated mast cells expressing tryptase are increased more than sevenfold throughout the cycle in endometrial polyps (n = 20) compared with normal endometrium. This novel finding has important implications for growth, development, and symptoms associated with polyps in many different tissues.