Objective-Transforming growth factor-β inhibits migration and proliferation of endothelial and smooth muscle cells.Endoglin is a transmembrane receptor for transforming growth factor-β1 and transforming growth factor-β3. Endoglin is released into blood as a soluble form (soluble endoglin [sEng]), but plasma sEng levels in patients with coronary artery disease (CAD) have not been elucidated. Approach and Results-We measured plasma sEng levels in 244 patients undergoing coronary angiography. The severity of coronary atherosclerosis was evaluated as the numbers of >50% stenotic vessels and segments. CAD was found in 147 patients, of whom 55 had 1-vessel, 42 had 2-vessel, and 50 had 3-vessel disease. Compared with 97 patients without CAD, 147 with CAD had lower sEng levels (median 4.04 versus 4.37 ng/mL; P<0.005). A stepwise decrease in sEng levels was found based on the number of stenotic vessels: 4.37 in CAD(−), 4.23 in 1-vessel, 4.13 in 2-vessel, and 3.74 ng/mL in 3-vessel disease (P<0.005). sEng levels inversely correlated with the number of stenotic segments (r=−0.25; P<0.001). In multivariate analysis, sEng was an independent factor for 3-vessel disease and CAD. Odds ratios for CAD and 3-vessel disease were 0.97 (95% confidence interval, 0.95-0.99; P<0.02) and 0.96 (95% confidence interval, 0.93-0.99; P<0.01) for a 0.1 ng/mL increase in sEng levels, respectively. Conclusions-Plasma sEng levels were low in patients with CAD, especially 3-vessel disease, and were inversely associated with the severity of coronary atherosclerosis. The study sponsors had no role in the design, analysis, and interpretation of the study. The online-only Data Supplement is available with this article at http://atvb.ahajournals.org/lookup/suppl