S.D. Codified Laws § 36-2-19
It is unprofessional for a practitioner of the healing arts to refer a patient to an unaffiliated health care facility in which the practitioner, the practitioner's immediate family, or a corporation or limited liability company owned in whole or in part by the practitioner has a substantial financial interest without disclosing the general nature of this interest to the patient, the patient's parent or guardian, at the time of the referral. As used in this section, a substantial financial interest is an ownership interest of twenty-five percent or more, a direct creditor's interest in twenty-five percent or more of the facility's debt, or a direct lessor's interest in twenty-five percent of more of a lease to the facility. Immediate family means the spouse and children of the practitioner. Such conduct constitutes grounds for discipline by the practitioner's appropriate licensing board.
SDCL 36-2-19