Pulmonary arterial hypertension (PAH) is a haemodynamic and pathophysiological condition with restricted flow through the pulmonary arterial circulation. In pulmonary hypertension, right ventricular hypertrophy and diastolic dysfunction can lead to an increase in atrial strain, fibrosis and dilation, which cause inhomogeneous atrial conduction. Interlead variation in P-wave duration is called P-wave dispersion (PwD), which is an electrocardiographic parameter that can be used to predict atrial arrhythmias. Our aim was to investigate the relationship between PwD, functional capacity, and invasive and non-invasive haemodynamic parameters of patients diagnosed with PAH. Between 2015 and 2017 we enrolled 33 patients admitted to our in-patient clinic and diagnosed with PAH, and 32 healthy individuals for the control group. Details of these patients at the time of diagnosis were analysed, including gender, age, physical examination, electrocardiogram (ECG), echocardiography, six-minute walk test distance (6MWD), haemodynamic parameters and blood tests for biochemical markers that are correlated with clinical severity. Statistical analyses were performed using SPSS version 20.0 (SPSS Inc, Chicago, Illinois, USA). Statistical significance was taken as p < 0.05. In the forward stepwise multiple linear regression analysis, PwD and mean pulmonary artery pressure determined by right heart catheterisation were independently related to the functional capacity tested by the 6MWD (p < 0.02 and p < 0.01, respectively). PwD can easily be calculated from a surface ECG to indirectly estimate the functional status and prognosis of the patient with PAH.