Frequency-modulated (FM) long-term ECGs, leads CM5 and CC5, were analysed to define the features which characterize silent ischemia in stable angina. Of 40 consecutively examined out-patients with angiographically proven coronary heart disease 22 had more than three ischemia-type episodes, of which at least one each was symptomatic or asymptomatic. Compared with the asymptomatic ones, the symptomatic episodes had significantly more ST deviations of greater than or equal to 0.15 mV (45.5% vs 17.3%; P less than 0.001) or greater than or equal to 0.2 mV (24.2% vs 5.8%; P less than 0.001). Simultaneous ST deviations in both leads were registered more frequently during symptomatic than silent episodes (43.9% vs 22.4%; P less than 0.01). The incidence of prolonged (greater than or equal to 10 min) ST-deviations was not significantly different during symptomatic and asymptomatic episodes (47.0% vs 40.4%; P greater than 0.05). While only 29.7% of all identified episodes were accompanied by symptoms, interindividual comparison indicated that in 63.6% of patients the ischemic episode with maximal ST deviation was symptomatic. It is concluded from these results that in stable angina the occurrence of anginal symptoms is influenced by the intensity and extent of the myocardial ischemia.