Rotational atherectomy catheters can ablate atheroma and restore patency in atherosclerotic arteries but little is known about the resultant particulate debris or its effect on myocardial blood flow. Therefore, we examined particulate debris number and size distribution after ablation of human necropsy aorta utilizing a sophisticated laser fluorescence and light scattering particle counter. Almost 90% of the particles were less than 12 microns in size and 80% were less than 5 microns in size. To determine the acute in vivo effects of particle debris on myocardial blood flow, carefully graded injections of particulate were infused into the left circumflex coronary artery of dogs and blood flow was measured using radio-labeled microspheres. Preliminary results indicate that only very large doses of calcified particulate caused deleterious hemodynamic effects. These studies will be continued to formalize our understanding of the safe particle burden which could be tolerated by the coronary microcirculation without significant effects on myocardial blood flow and/or left ventricular hemodynamics.