Acute myocardial infarction in systemic lupus erythematosus may be due to an atheromatous or arteritic process. Confirmation of the latter etiology has previously been made only at postmortem examination. A 45-year-old white woman with known systemic lupus erythematosus developed anginal pain and multiple episodes of acute myocardial infarction. During this period, there was serologic but no other clinical evidence of active systemic lupus erythematosus. Serial coronary angiographic studies were strongly suggestive of an arteritic process based upon (1) a saccular aneurysm with no obstructive lesions in a coronary artery supplying an area of recent transmural myocardial infarction and (2) the development of significant obstructive lesions in a previously normal coronary artery over a period of 18 days. This case illustrates the difficulties in distinguishing between atherosclerosis and arteritis using a single coronary angiographic study. The distinction is significant because of the different therapeutic interventions required.