The relationship between pulmonary haemodynamics and values of various respiratory function tests was studied in patients with mild chronic obstructive pulmonary disease (COPD) and the following results were obtained. (1) The value of mean pulmonary artery pressure (mPAP) at rest in COPD patients was slightly elevated to 19·4 mmHg on average compared with our control value of less than 18 mmHg. (2) Analysis of the data of 11 routine respiratory function tests in 88 COPD patients extracted two principal components: an index of the expiratory function and an index for overinflation of the lung. (3) In individual patients, mPAP expressed the severity of pulmonary circulatory disorder roughly inverse to the factor score of the first principal component (index of expiratory function) but not to that of the second principal component (overinflation of the lung). (4) Discriminant analysis was performed in all 88 COPD patients according to data from the 11 respiratory function tests. The probability of mPAP being above or below 18 mmHg was 18·2%. (5) The relationship between the predicted EPOI value and the factor score was similar to that between mPAR and the factor score. EPOI (exercise pulmonary artery pressure-oxygen consumption index) was calculated with the following equation: EPOI = (mPAP ex #-mPAP rest)/(( V̇o 2ex- V̇o 2rest)/BSA ##). On the other hand, EPOI pred was calculated with the prediction equation obtained from multiple linear regression (dependent variable; EPOI, independent variable; respiratory function).