A study was undertaken to determine whether measurements of radiological indices from postero-anterior chest X-rays were useful in predicting pulmonary artery hypertension. Measurements of the transhilar (THD) and pulmonary lobar distances (PLD) as well as the width of the descending branch of the right pulmonary artery (DB) were made from X-rays of 100 patients with chronic obstructive pulmonary disease. For these patients the forced expiratory volume in 1 s was 1.2 +/- 0.6 l (group mean +/- SD), the arterial PO2 was 62.2 +/- 14.5 mmHg and the mean pulmonary artery pressure (PAP) was 26.7 +/- 11.9 mmHg. Considerable differences in the measurement of THD and PLD were found between and within observers whereas the measurement of DB was more reproducible. DB was better correlated (r = 0.59, P less than 0.001) with PAP than were THD and PLD. Using a stepwise multiple regression procedure including other physiological variables, it was found that DB and arterial PO2 were the only significant (P less than 0.05) predictors of PAP, together accounting for 48% of the variation in PAP, with DB being the more important predictor.