To examine whether measurements of lung thallium-201 (Tl-201) uptake with single photon emission computed tomography (SPECT) following exercise or dipyridamole infusion provide complementary information as reported in planar imaging, we examined 140 patients undergoing exercise or dipyridamole Tl-201 SPECT and coronary arteriography for the diagnosis of coronary artery disease (CAD). On the reconstructed coronal tomographic image, regions of interest were placed over the left upper lung field and the left ventricular myocardium. Lung to heart ratio (L/H ratio) was calculated as a fraction of average Tl-201 counts per pixel in the lung divided by those in the myocardium. An L/H ratio of larger than a value of mean +2 standard deviation, derived from patients without significant coronary stenosis, was considered as abnormal. The patients with abnormal L/H ratio on exercise Tl-201 SPECT had larger extent and severity scores of Tl-201 defects on the initial image, lower left ventricular ejection fraction, higher end-diastolic pressure and higher incidence of multi-vessel CAD than those with the normal ratio. However, in dipyridamole studies, abnormal L/H ratio did not relate to scintigraphic or hemodynamic severity. These data confirmed that increased lung Tl-201 uptake after exercise suggested the presence of extensive CAD or left ventricular dysfunction. However, L/H ratio might be less useful in dipyridamole studies. Thus, quantitative measurements of L/H ratio with exercise but not dipyridamole SPECT could provide additional information related to the involved cardiac pump performance in CAD.