Circadian periodicity was examined in 68 patients with chronic stable angina and in 9 patients with Prinzmetal angina. The frequency and duration of transient ischemic episodes were determined from analysis of 1 or more 24-hour Holter recordings by the compact analog technique. Ninety percent of the episodes in both syndromes were silent; 80% of the episodes of Prinzmetal angina were associated with ST-segment elevation and all episodes of chronic stable angina had ST-segment depression. Ischemic episodes were shorter (3 +/- 2 vs 18 +/- 23 minutes, p less than 0.0005) but more frequent (21 +/- 18 vs 6 +/- 4 per 24 hours, p less than 0.0001) in patients with Prinzmetal angina than in those with chronic stable angina. In patients with chronic stable angina, both silent and painful episodes had a peak occurrence in the morning and early afternoon hours (between 8 AM and 3 PM); the fewest episodes were between 1 AM and 5 AM. This distribution was not random by chi-square test (p less than 0.001). Cosinor analysis of ischemic episodes periodicity showed the acrophase at 1 PM, which was not different from that (3 PM) of the circadian rhythmicity for heart rate. In case of Prinzmetal angina, the acrophase of heart rate changes was at 5 PM, but a clear periodicity in the distribution of the ischemic episodes was not found. These differences in the circadian periodicity may reflect differences in the mechanism of ischemia in chronic stable angina and in Prinzmetal angina and are likely to be of therapeutic significance.