A prospective study of 18 patients admitted to hospital for acute pulmonary embolism confirmed the reliability of continuous wave cardiac doppler as a non-invasive method of evaluating systolic pulmonary artery pressures. These pressures were calculated by applying the simplified Bernoulli equation to the maximal velocity of regurgitant tricuspid flow and compared with the results of cardiac catheterisation and angiography, the percentage of vascular obstruction being assessed using Miller's index. The correlations between the two methods were good, r = 0.96; p less than 0.001, with a standard error of +/- 5.2 mmHg. The correlations between the velocity of tricuspid flow and the percentage of obstruction were less significant (r = 0.65; p less than 0.005) but improved when patients with pre-existing cardiopulmonary disease were excluded. This technique of non-invasive assessment of haemodynamic parameters also helps in evaluating the underlying pathology; tricuspid regurgitation with velocities greater than 3.5 m/s is associated with pre-existing chronic cor pulmonale, information of prognostic interest which would guide therapeutic management.