Background. In-stent restenosis is still a significant problem, and early diagnosis of in-stent restenosis after coronary intervention is an important issue. We sought to clarify whether 1) Doppler echocardiography is feasible to visualize instent flow signal, and 2) in-stent coronary restenosis can be detected by transthoracic Doppler echocardiography (TTDE).Methods. 1) A Doppler flow phantom with stent-implanted tubing was used. Under color Doppler flow mapping, we measured in-stent flow velocities by pulsed Doppler echocardiography and Doppler guidewire. 2) TTDE was performed in 128 patients after stent implantation in the left anterior descending artery (LAD) (18 with in-stent restenosis, group R; 110 without restenosis, group N). After searching for localized aliasing with color Doppler echocardiography, coronary flow velocities were measured at the alias (stenotic) site and at the pre-stenotic site.Results. 1) In-stent flow was visualized by color Doppler echocardiography, and mean velocities that were measured by pulsed Doppler echocardiography and Doppler guidewire showed good agreements (y=0.86×+6.8, r=0.99). 2) The LAD was visualized in 118 patients. Localized aliasing was detected in 16 patients in group R and 38 patients in group N. The pre-stenotic to stenotic MDV ratio was significantly lower in group R than group N (0.43±0.07 vs. 0.67±0.13, p<0.001). Localized aliasing with the pre-stenotic to stenotic mean diastolic velocity ratio < 0.5 had a sensitivity of 94% and a specificity of 89% for the detection of in-stent restenosis.Conclusions. Doppler echocardiography could detect coronary flow signal directly through the coronary stent in vitro. TTDE is feasible for noninvasive detection of in-stent coronary restenosis in patient after coronary stent implantation.