The previous definition of exercise pulmonary hypertension (PH) with a mean pulmonary artery pressure (mPAP) >30 mmHg was abandoned because healthy individuals can exceed this threshold at high cardiac output (CO). We hypothesised that incorporating assessment of the pressure-flow relationship using the mPAP/CO ratio, i.e. total pulmonary resistance (TPR), might enhance the accuracy of diagnosing an abnormal exercise haemodynamic response.Exercise haemodynamics were evaluated in 169 consecutive subjects with normal resting mPAP ≤20 mmHg. Subjects were classified into controls without heart or lung disease (n=68) versus patients with pulmonary vascular disease (PVD) (n=49) and left heart disease (LHD) (n=52).TPR and mPAP at maximal exercise produced diagnostic accuracy with area under the receiver operating curve of 0.99 and 0.95, respectively, for discriminating controls versus patients with PVD and LHD. The old criterion of mPAP >30 mmHg had sensitivity of 0.98 but specificity of 0.77. Combining maximal mPAP >30 mmHg and TPR >3 mmHg·min·L(-1) retained sensitivity at 0.93 but improved specificity to 1.0. The accuracy of the combined criteria was high across different age groups, sex, body mass index and diagnosis (PVD or LHD).Combining mPAP >30 mmHg and TPR >3 mmHg·min·L(-1) is superior to mPAP >30 mmHg alone for defining a pathological haemodynamic response of the pulmonary circulation during exercise.