We analyzed the diurnal distribution of the onset of infarction in 636 patients with acute myocardial infarction (MI) and compared it with the circadian variation of blood pressure in 57 patients with coronary artery disease (CAD). In addition, we studied the modification of the circadian blood pressure variation during treatment with antianginal medications in 20 patients with CAD. A marked diurnal periodicity (p < 0.05) was observed for the onset of MI, with peaks seen in the late morning, late evening, and very early morning. The blood pressure in the patients with CAD was elevated in the morning, reduced in the late evening, and was the lowest in the very early morning. The peaks of onset of infarction temporally corresponded to the characteristic feature of blood pressure profile observed in the patients with CAD, that is, the morning rise, the late evening decline, and the very early morning reduction. Antianginal medications significantly reduced the blood pressure, not only during the day (p < 0.02) but also at night (p < 0.05). These observations suggest that the decline in blood pressure, as well as the morning surge in blood pressure, may be closely related to the onset of MI. Therefore, when treating patients with CAD with antianginal medications which can potentially reduce blood pressure, the effects on the circadian variation of blood pressure should be considered.