Endometriosis is a chronic disease characterized by ectopic deposits of endometrial glands and stroma located outside the uterus. Women with symptomatic endometriosis may experience premenstrual bleeding or staining, pain and other physical sensations, as well as other symptoms dependent upon the stage and location of the endometrial implants. We discuss the particular implications of these symptoms for women who observe the part of Jewish law known as hilkhot niddah. The laws of niddah, also known as taharat hamishpahah (family purity), dictate the timing of the physical relationship between a married couple. These laws proscribe any physical contact between the couple during the time that the wife has the status of niddah. This status is obtained by any uterine bleeding that is not caused by injury. Menstruation is the most common cause of the niddah status, but niddah and menstruation are not synonymous. Since, to the best of our knowledge, there is no written discussion of the specific implications of endometriosis for this population, we discuss the relevant halakhic and medical literature and hope that such analysis will facilitate efforts to assist the observant couple in gaining regular niddah-free segments of time.