Two cases of high-output heart failure associated with pulmonary hypertension are presented. In a 32-year-old man with hyperthyroidism, pulmonary hypertension subsided after antithyroid therapy. Increased pulmonary blood flow and elevated left ventricular end-diastolic pressure were suspected as the causes of pulmonary hypertension. In a 51-year-old man with cardiac beriberi, the pulmonary hypertension improved after thiamine administration. We suspect that increased pulmonary blood flow, elevated left ventricular end-diastolic pressure, and probably pulmonary vasoconstriction were the causes of pulmonary hypertension. Thus, high-output heart failure should be considered as a possible cause of pulmonary hypertension and right ventricular failure.