Whether progestins, particularly medroxyprogesterone acetate (MPA), attenuate the cardiovascular benefits of postmenopausal estrogen replacement therapy (ERT) has been controversial for over a decade. Concerns related first to findings that MPA attenuated increases of high density lipoprotein cholesterol (HDLC) concentrations of postmenopausal women compared to conjugated equine estrogen (CEE) alone. That observation was followed by early cynomolgus monkey studies that suggested MPA decreased estrogen's cardiovascular benefits (vascular reactivity and coronary artery atherosclerosis inhibition). In a more recent and larger trial with cynomolgus monkeys, no differences were seen in the coronary artery atherosclerosis protective effect of CEE when MPA was co-administered (HRT). The lack of attenuation of ERTs benefits by progestins has also been seen in at least three studies of carotid artery intima-media thickness (IMT) of postmenopausal women. Additionally, the majority of studies of vascular reactivity of postmenopausal women have not found differences when CEE is given alone or with MPA. Seven observational studies of cardiovascular outcomes of postmenopausal women permit separate consideration of ERT versus HRT use; there is no evidence of attenuation of ERTs benefits by progestin use. In conclusion, it is evident that the current experimental, clinical, and observational data do not provide evidence that progestins attenuate estrogen's cardiovascular benefits.