Fifty two patients with coronary artery disease underwent repeat coronary arteriographic studies separated by 2-108 (mean 51) months of medical treatment. The results were compared and correlated with symptoms to determine the nature of the progression of coronary atherosclerosis. The condition appeared to progress episodically in the proximal segments of the coronary arteries and in relation to the abrupt development of new symptoms or acute coronary events such as unstable angina or myocardial infarction. Thirty four of 105 (33%) of the pre-existing stenoses showed evidence of progression. Progression to total occlusion was uncommon (13) except for stenoses greater than 90% (six out of 18). New lesions frequently occurred (37) in previously normal segments of the arteries; most of these were stenoses greater than 90% (13) or total occlusions (12). Rapid progression of a mild lesion and new lesions occurred in the form of smooth intimal protrusions into the arterial lumen. Intimal haemorrhages are the likely explanation for these intimal encroachments and also for the episodic nature of the progression of coronary artery disease. Coronary atherosclerosis does not progress gradually in a linear fashion, and local anatomical factors appear to play a dominant role in the natural history.