In patients with all types of pulmonary hypertension, especially idiopathic pulmonary hypertension, pulmonary vasospasm may be involved in the development of pulmonary hypertension. Acute vasodilator drug tests are an effective means of screening these patients, so a positive acute vasodilator drug test suggests that a significant number of small pulmonary arteries in the pulmonary circulation are in spasm in that patient. Recent studies have confirmed that treatment with calcium antagonists in patients with positive test results can result in a significant improvement in prognosis. In addition, patients with a greater than 50% decrease in total pulmonary resistance index after the first admission for an acute vasodilator drug test have a better prognosis than patients with a lower response. For these reasons, it is important to evaluate the acute vasodilator test when performing the first right heart catheterization for a patient.