Many workers have tried to define echocardiographic criteria of left ventricular function. The Q-MVC/AVC-E index (ratio of the interval between the start of the Q wave and mitral valve closure and the interval between aortic valve closure to the E point of maximal mitral opening) was proposed to quantify mean pulmonary capillary pressure. The valve of this index was assessed in 62 consecutive patients by 67 simultaneous measurements of mean pulmonary capillary pressure by catheterisation and M mode echocardiography. After the exclusion of pacemaker patients and those with arrhythmias (atrial fibrillation or flutter) or conduction defects, a close correlation was observed between the two methods. In 66% of patients in sinus rhythm without a conduction defect, the difference between the results obtained by the two methods was less than or equal to 2 mmHg. In 96% of cases, the difference was less than or equal to 5 mmHg (r = 0.97; n = 52, p less than 0.0001). In addition, the results of the echocardiographic measurements were reproducible in the same patient when the electrocardiographic parameters were the same. However, there is a large number of electrical and haemodynamic factors which can affect the two terms of this index, and a good quality echocardiographic recording in essential for the measurement of these intervals. Therefore, these theoretical and practical limitations oppose the substitution of echocardiography for right cardiac catheterisation for the determination of mean pulmonary capillary pressure.