A non-invasive and easily repeatable method of pulmonary gas exchange evaluation, using measurements of stable carbon monoxide transfer capacity (DLCO) and alveolar-capillary ductance ( DuaCo ), was compared to a complex, invasive method of pulmonary vascular bed evaluation (pulmonary angiography) in 17 patients during the first 24 hours of pulmonary embolism. DLCO and DuaCo measurements were subsequently repeated in several of the patients during the course of the disease. The study showed that DLCO was lowered in both massive (Walsh's angiographic index 12.9 +/- 1.9) and submassive (Walsh's index 4.1 +/- 1.3) pulmonary embolism, with values of 7.0 +/- 2.5 and 7.0 +/- 1.8 ml/min/mm Hg respectively. It also showed a highly significant correlation between the degree of vascular obstruction, as determined by Walsh's angiographic index, and the reduction of alveolar-capillary gas exchange, as evaluated by DuaCo (r = 0.80; p less than 0.001). A progressive improvement in the pulmonary capillary bed perfused could be suspected from the results of DuaCo monitoring during the course of embolism.