In this Committee Opinion, the American College of Obstetricians and Gynecologists Committee on Ethics presents various ethical considerations and arguments relevant to both prefertilization and postfertilization techniques for sex selection. The principal medical reason for sex selection is known or suspected risk of sex-linked genetic disorders. Other reasons sex selection is requested are personal, social, or cultural in nature. The Committee on Ethics supports the practice of offering patients procedures for the purpose of preventing serious sex-linked genetic diseases. However, the committee opposes meeting requests for sex selection for personal and family reasons, including family balancing, because of the concern that such requests may ultimately support sexist practices. Because a patient is entitled to obtain personal medical information, including information about the sex of her fetus, it will sometimes be impossible for health care professionals to avoid unwitting participation in sex selection.