As an introduction to a prospective study on the complex non-invasive estimation of pulmonary arterial pressure, the mortality experience of 834 patients with chronic respiratory disease was analysed historically. In patients with chronic obstructive lung disease, chronic fibrosing alveolitis and pulmonary tuberculosis, the relationships between mortality and values of ten physiological variables were examined. The most important variable as regards mortality in chronic obstructive lung disease was FEV1.0; after controlling for FEV1.0, pulmonary arterial pressure, arterial oxygen and carbon dioxide pressures were all strongly related to mortality. By contrast, in fibrosing alveolitis, values of FEV1.0 had little relation to mortality, and pulmonary arterial pressure was the most important variable, followed by FVC and FEV/VC. In patients with tuberculosis, arterial oxygen and pulmonary arterial pressure had the strongest relationship with mortality. Pulmonary arterial pressure was therefore strongly related to mortality in all three conditions; FEV1.0 had its highest predictive power in chronic obstructive lung disease and arterial oxygen saturation in tuberculosis. In chronic obstructive lung disease, pulmonary arterial pressure had a stronger influence in relation to mortality of younger than of older patients.