In order to evaluate Doppler echocardiography (DEC) used for cardiac output (CO) measurement, 22 patients (7 with coronary heart disease, 6 with dilatation cardiomyopathy, 5 with primary pulmonary hypertension and 4 with rheumatic heart disease) were studied. In all patients, invasive CO determination was performed by right heart catheterization, using thermodilution (T). In DEC, CO was calculated as CO = IV.S.HR, where IV was integral velocity, estimated on the basis of flow areas under planimetric curves, S was aortic root cross-section area, and HR was heart rate. A close correlation was demonstrated between DEC and T results (r-0.74). Only in one case with severe aortic atherosclerosis was there a significant difference between the data. This case excluded, the correlation becomes even closer (r-0.86). It is concluded that: 1) DEC is a valuable and precise method of CO assessment, 2) DEC may be used to monitor treatment efficacy, and 3) its limitation is aortic lesions.