This study assessed both group and individual variability of ECG exercise stress test in patients with effort angina. Forty-five untreated patients with typical effort angina, without evidence of spontaneous angina, with a positive exercise stress test (ST depression greater than 0.2 mV) and angiographically documented coronary artery disease were studied. Four multistage exercise stress tests were performed, two in the morning and two in the afternoon, on two consecutive days. Forty-four patients completed the protocol for a total of 176 exercise stress tests. For each exercise stress test the following parameters were analyzed: time to 0.15 mV ST segment depression (time to ischemia); rate-pressure product at ischemia (ischemic threshold); slope and intercept of the linear regression between rate-pressure product and time of exercise. Silent effort ischemia was largely prevalent: 21 patients (48%) experienced chest pain in all four tests, but only seven showed a consistent time relationship between pain and ECG changes. Symptomatic patients did not appear different from the asymptomatic ones in terms of clinical and angiographic data. When group data were analyzed for each parameter the four exercise stress tests appeared reproducible. In contrast, when individual variability of each parameter was computed as the percentage difference between range (maximum--minimum) and maximal value obtained in the series of four exercise stress tests, a large variability was detected. Variability of time to ischemia, was 27.2 +/- 17.4%. This resulted from a random combination of variability in ischemic threshold (19.1 +/- 9.2%), slope (28.4 +/- 12.8%) and intercept (22.7 +/- 10%).(ABSTRACT TRUNCATED AT 250 WORDS)