This Note uses ratings of appropriateness derived from an expert physician panel to measure how appropriately physicians in 1981 performed coronary angiography in a randomly selected, community-based sample of cases in the Medicare population. The authors studied three large geographic areas in three states, representing regions of high and low use. The high-use site had fewer procedures classified as appropriate (72 percent) than either low-use site (77 percent and 81 percent). Over all sites, 17 percent of procedures were classified as inappropriate. Patients in the high-use site were older, had less severe angina, and were less intensively medically treated than patients in either of the low-use sites. Patients without angina who had not undergone exercise testing constituted the most common subgroup of inappropriate cases. Although overall differences in appropriateness were not large, practice differences do exist. The analysis of practice differences among study sites provides the clinical basis for understanding the small, but significant, differences in the appropriateness of use of coronary angiography. The finding of 17 percent inappropriate use may be cause for concern.