Correlation has been established between the size of the main pulmonary arteries, as seen on the postero-anterior radiograph, and the pulmonary artery pressure. In this study the distance between the bifurcations of the right and left pulmonary arteries, referred to as the transpulmonary artery distance (TPD), has been used as a measure of the size of these vessels. In 293 patients with simple pneumoconiosis TPD was significantly correlated with height but not with stem-height, age, weight or forced expiratory volume. The mean TPD of patients with simple pneumoconiosis with or without chronic bronchitis was similar. Among 367 patients with complicated pneumoconiosis those with early fibrosis (A shadows) had a mean TPD similar to that found in simple pneumoconiosis, but those patients with extensive fibrosis (B or C shadows) had significantly greater values. The pathological changes of pulmonary hypertension in complicated pneumoconiosis are reflected in these measurements.