Six new labdane diterpenoids, preleosibirone A (1), 13 epi-preleosibirone A (2), isopreleosibirone A (3), leosibirone A (4), leosibirone B (5), and 15-epi-leosibirone B (6), were isolated from the leaves of Leonurus sibiricus. The absolute configurations of 1, 2, 5, and 6 were established by X-ray crystallographic analyses, and leosibirone A (4) was shown to be an artifact of the isolation process. Leonurus sibiricus L. (Lamiaceae) is a widely distributed medicinal plant in southern Siberia, China, Korea, Japan, Vietnam, and elsewhere in Southeast Asia. It is commonly known as "motherwort" in Asian countries. It has been introduced to Mexico and named "marihuanilla" (Spanish "little marijuana"). The plant is used traditionally for various ailments, such as menstrual irregularities, high blood pressure, blood stasis, heart disorders, and dysentery.1-6 Leaves harvested from the flowering plant are dried and smoked as a marijuana substitute. The effects are described as mildly narcotic or cannabislike.7 Previous phytochemical investigations resulted in the isolation of alkaloids, flavonoids, iridoids, and phenylpropanoid glycosides.8-12 In addition, there are several reports of labdane diterpenoids from L. sibiricus. 8,13-16 Herein, we report the isolation and structure elucidation of six new labdane diterpenoids (1-6) from this species.