In 100 patients with a normokinetic left ventricle 201-thallium imaging following administration of dipyridamole (0.50 mg/kg body wt. i.v.) showed a sensitivity of 67% (41/61) and a specificity of 95% (37/39) in diagnosing coronary artery disease. Exercise stress testing gave nearly equal sensitivity values (36/58 = 62%) with only slightly lower specificity (32/39 = 85%). The rate of true positive 201-thallium findings increased with the number of narrowed vessels (one vessel disease 50%, two vessel disease 63%, three vessel disease 94%); the extent of coronary artery disease was almost regularly underestimated with the scintigraphic method. Because of the low sensitivity a negative finding cannot exclude the presence of coronary artery disease, but the existence of three vessel disease can be considered improbable in patients in whom thallium scintigraphy findings are negative. Analysis following Bayes' theorem revealed the method to have a low diagnostic value in patients with either very low or high pretest probability for the disease; either a positive or a negative finding is of most use in a patient with low or average pretest probability of the disease (20-60%).