Over the last few years, patterns have emerged regarding the daily (circadian), weekly (circaseptan), and yearly (circannual) variation in the incidence of acute coronary syndromes (ACS). Peaks of incidence occur in the morning, on Mondays, and in winter. There is a difference in the pattern of incidence in different subgroups such as diabetics and smokers, which, along with the incidence alteration seen with aspirin and beta blockers, gives us some potential understanding of underlying mechanisms. Recent advances in the study of endothelial function, cytokine biology, and adhesion molecules have led to new insights into the way that natural fluctuations in these systems may affect ACS incidence. It is hoped that understanding these developments will lead to therapeutic advances in ACS prevention.