To open up the possibility of exact clinical diagnoses of chronic cor pulmonale, we performed thallium-201 myocardial perfusion scintigraphy on patients with chronic pulmonary disease. From the thallium activity in the left ventricle (plus ventricular septum) and right ventricle counted scintigraphically, we determined the thallium activity ratio of left to right ventricle (TAR). The intra- and inter-observer variations of this ratio were negligible. We compared TAR with pulmonary hemodynamic findings and with left to right ventricular mass ratio measured at autopsy, and obtained the following results. 1) Left to right ventricular mass ratio determined in 11 autopsied patients by the method of Fulton et al. was closely correlated with TAR values (r = 0.90, p less than 0.001). 2) In patients with chronic pulmonary disease, there were highly significant exponential inverse correlations between TAR and pulmonary arterial mean pressure, total pulmonary vascular resistance and right ventricular systolic pressure the the coefficients of -0.75, 0.72 or -0.77, respectively (p less than 0.001). These results suggested the possibility that the severity of pulmonary circulatory disturbance may be ascertained non-invasively by TAR measurement. Thus, thallium-201 myocardial perfusion scintigraphy is able to reveal non-invasively right ventricular hypertrophy in patients with chronic pulmonary disease, and it might be a useful means of clinically diagnosing chronic cor pulmonale. If TAR obtained from scintigrams is smaller than 2, the patient concerned may possibly have right ventricular hypertrophy and he or she may be diagnosed as having chronic cor pulmonale.