Abstract It is now possible experimentally to measure myocardial blood flow of the beating heart using a helium-neon (HeNe) laser Doppler flowmeter. A myocardial probe was redesigned to reduce its size and weight, and a method devised of fixing the probe to the beating cardiac surface to allow its clinical application. This modified laser flowmeter was used on 36 patients with ischaemic heart disease to measure myocardial blood flow before and after revascularization. Flow was measured in the right and left ventricles while patients were in a haemodynamically stable state as determined by electrocardiography, heart rate, blood pressure and double product (heart rate × systolic blood pressure). No significant difference was found between the mean(s.e.m.) preoperative and postoperative flow volume at the anterior wall of the right ventricle (77(15) versus 81(12) ml/min per 100 g), which did not undergo revascularization, but mean(s.e.m.) myocardial blood flow at the ischaemic left ventricle increased significantly (from 68(15) to 88(13) ml/min per 100 g: P<0.01). There was also no significant difference between preoperative and postoperative values of haemodynamic parameters of coronary blood flow. In conclusion, a means to measure myocardial blood flow with HeNe laser Doppler flowmetry has been devised which shows coronary artery bypass grafting to increase myocardial blood flow in the ischaemic myocardium.