Heart rate (HR) at rest is an important marker of prognosis in heart failure, but has not been addressed in pulmonary arterial hypertension (PAH). To determine the prognostic value of HR at rest in patients with PAH, we retrospectively analyzed 140 consecutive patients with idiopathic PAH. Electrocardiogram (ECG)-derived HR at rest was evaluated as a potential predictor of adverse prognosis (death or lung transplantation), in addition to World Health Organization functional class, 6-minute walk distance, and hemodynamics before and approximately 1 year and 2 years after initiation of PAH treatment. During follow-up, 49 patients (35%) died, and 5 patients (4%) underwent lung transplantation. Before treatment initiation and after 1 year and 2 years of treatment, respectively, a higher HR at rest was an independent predictor of adverse prognosis (hazard ratios per 10-beats/min increase 1.76, 95% confidence interval 1.42 to 2.18, 2.31, 95% confidence interval 1.58 to 3.38, 2.1, 95% confidence interval 1.39 to 3.19, respectively, p <0.001 for all). Change in HR between the first and last ECG also independently predicted prognosis (hazard ratio per 1-beat/min increase 1.03, 95% confidence interval 1.01 to 1.06). In conclusion, a higher HR at rest and an important increase in HR at rest during follow-up signify a considerable risk of death in patients with PAH. ECG-derived HR at rest is an important marker of prognosis and should be assessed before and at frequent intervals after initiation of treatment for PAH.