Pulmonary circulation hemodynamics was studied by microcatheterization at rest and during exercise in 16 workers with asbestosis. There was pulmonary arterial hypertension owing to increased total pulmonary resistance even in the early stages of pneumoconiosis, sometimes without impairment of lung function. In two cases, a partial reduction in pulmonary hypertension and total pulmonary resistance was obtained with aminophylline. The pathogenesis of pulmonary circulation impairment has been attributed in previous work to periarteriolar fibrosis originating in the lymphatic vessels, until now insufficiently emphasized as the anatomical basis of these physiological alterations.