A study was undertaken to examine the value of rapid atrial pacing and thallium-201 scintigraphy in the evaluation of ischemic heart disease. Ten subjects with angiographically normal coronary arteries and 31 patients with coronary artery disease were studied. Six of these patients were studied after coronary artery bypass surgery. Coronary ischemia or infarction could be diagnosed from the ECG alone (rest and atrial pacing) in 87% of the patients with a greater than or equal to 50% narrowing of a coronary artery. The ECG and scintiscan (at rest and pacing) together had a sensitivity of 100%. The ECG could not identify the site of stress-induced ischemia. Rapid atrial pacing with scintigraphy identified 91% of those patients with a right coronary artery narrowing (greater than 50%), 85% with a narrowing of the left anterior descending coronary artery, but only 33% with a narrowing of the circumflex coronary artery. The scintiscan correctly identified 40% of arteries with a 50 to 89% narrowing, 75% with a 90 to 99% narrowing and 88% with a complete obstruction. The scintiscan predicted single- and double-vessel disease, but underestimated the number of vessels involved in patients with triple-vessel disease. This technique was also useful in assessing the patency of implanted coronary artery bypass grafts. Rapid atrial pacing is a simple and safe method of inducing ischemia, while thallium scintigraphy shows the site, extent and severity of coronary artery disease.