We developed a new method for calculating a phasic resistance (Rp) of left coronary artery in the diastole. The Rp was calculated by using the phasic coronary flow (Fp) and pressure (Pp) obtained as follows: Fp was obtained by a vessel-tracking pulse Doppler flowmeter which permits the sample volume to follow the movements of the vessel. Pp was noninvasively obtained by measuring the diameter change in carotid artery by an echo tracking technique, because it was proven by us that the waveform of carotid artery was quite similar to that obtained by the aortic pressure measured by catheter tip manometer. Rp was calculated in 22 patients: Normal (N)=7, Valvular heart disease (VD)=7, Hypertension (HT)=4, Dilated cardiomyopathy (DCM)=2, Others=2. The Rp in diastole was a minimum at the phase of peak flow and increased from the time of peak flow toward the late diastolic phase. In each group, meanRp was 43(N), 30(VD), 70(HT), 23(DCM), 47(Others), respectively. The minimum Rp was decreased by administration of Nitroglycerin (NTG) and its reduction ratio (Rp post/Rp pre) was less in VD group (0.52) than in N group (0.32) CONCLUSION: We measured coronary arterial resistance noninvasively and its change by administration of NTG could be measured noninvasively. The coronary arterial resistance was a minimum at the phase of peak flow and it was increased from the peak flow portion toward the late diastole in each patient group.