['Tamoxifen has mixed estrogen agonist and antagonist properties in estrogen-regulated tissues. Its effect on the cardiovascular system is not well defined. We carried out a study to investigate the effect of tamoxifen on peripheral vascular endothelial function.', 'Three groups of postmenopausal women (median age, 56 years; range, 39 to 69 years) with breast cancer were studied. Patients in group 1 (n = 10) were newly diagnosed with breast cancer and studied before and after 4 weeks treatment with tamoxifen. Group 2 women (n = 6) had been receiving long-term tamoxifen (3 to 5 years) and were studied while taking tamoxifen and 4 weeks after stopping it. The final group of 6 subjects were in remission from primary breast cancer and were not receiving or had previously received tamoxifen. Ultrasound assessments of endothelial function were done before and 4 weeks after the initiation or discontinuation of tamoxifen with the nontreatment group acting as control. All ultrasound imaging was made by a single investigator blinded to the therapeutic status of the subject. Brachial artery diameter was measured by ultrasound at baseline and 1 minute after reactive hyperemia. Flow-mediated reactivity (FMR) was defined as percent change in artery diameter from baseline 1 minute after reactive hyperemia.', 'There was no change in FMR in patients before compared with 4 weeks after starting tamoxifen (4.06% +/- 1.44% vs 3.97% +/- 1.20%, respectively, mean +/- standard error of the mean [SEM], P =.97). There was no significant change in FMR on withdrawal from tamoxifen (1.84% +/- 1.98% vs -0.42% +/- 1.44% on tamoxifen vs off tamoxifen, mean +/- SEM, P =.36). FMR in subjects taking tamoxifen was no different from the control group (3.17% +/- 1.05% vs 3.16% +/- 0.91%, respectively, mean +/- SEM, P =.995).'] Is tamoxifen detrimental to endothelial function in postmenopausal women with breast cancer?