Lung thallium-201 uptake during exercise emission computed tomography. To test whether the analysis of lung uptake is worth adding to the interpretation of exercise thallium-201 (201TI) emission computed tomography (ECT), a lung/heart ratio of 201TI uptake was measured from an anterior image during ECT in 25 clinically normal (Group 1), 91 angiographically normal (Group 2), and 265 subjects with coronary artery disease (Group 3). The mean +2 s.d. of ratios in Groups 1 and 2 were the same (0.37). In Group 3, 80 subjects (30%) with increased ratios (greater than 0.37) had higher frequencies of prior infarction and multi-vessel disease, more severe left ventricular dysfunction, and 201TI defects than 185 subjects with normal ratios. Subjects with markedly increased ratios (greater than 0.45) had three-vessel disease more frequently than those with slightly or moderately increased ratios (67% versus 14% or 35%). Thus the measurement of lung/heart ratios may provide information regarding the severity of coronary artery disease and should be added to the routine interpretation of exercise ECT. 