Serial measurement of lung volumes, effort-dependent expiratory flow rates, and steady-state diffusing capacity helped to establish the diagnosis of acute pulmonary embolism in 31 patients with angiograms demonstrating vascular obstruction. Also, analysis of results separated patients with pulmonary embolism alone from patients with pulmonary embolism and associated, pulmonary infarction, or from patients with coexistent chronic obstructive pulmonary disease and left ventricular failure (the most common diseases to mimic the clinical picture of pulmonary embolism in these patients). Not only were these methods helpful in establishing the diagnosis of acute pulmonary embolism, but also in evaluating the rate of resolution of vascular obstruction and the deficit in pulmonary function following resolution.