Background: To our knowledge, the sudden onset of symptomatic pulmonary hypertension after COVID-19 vaccination has not been described. Both cases presented here resulted in functional limitations and likely permanent organ damage. Case Summary: We report two cases of acute onset pulmonary hypertension in previously healthy adult males within three weeks of receiving the second dose of the Pfizer (BNT162b2) mRNA COVID-19 vaccine from different lots. Both patients experienced the sudden onset of severe fatigue and dyspnea on exertion with negative COVID-19 PCR testing. The diagnosis was made by serial transthoracic echocardiography in the first case and by both transthoracic echocardiography and right heart catheterization in the second. Discussion: Pulmonary hypertension is a serious disease characterized by damage to lung vasculature and restricted blood flow through narrowed arteries from the right to left heart. The onset of symptoms is typically insidious, progressive and incurable, leading to right heart failure and premature death. The World Health Organization (WHO) classifies pulmonary hypertension into 5 categories and recently re-defined as a resting mean pulmonary artery pressure greater than 20 mmHg. Sudden onset pulmonary hypertension would only be expected in the settings of surgical pneumonectomy or massive pulmonary emboli with compromise of at least 50% of the lung vasculature.