To evaluate pulmonary vasoreactivity in children and young adults with primary pulmonary hypertension, we performed cardiac catheterizations on nine patients with primary pulmonary hypertension (nine months to 23 years old) and made hemodynamic measurements: before and after infusing prostacyclin, and before and after administering sublingual nifedipine. Based upon the response to prostacyclin, patients were divided into responders and nonresponders using the following criteria: 20 percent or greater decrease in mean pulmonary arterial pressure; an increase in cardiac index; and no change, or a decrease in the pulmonary vascular resistance to systemic vascular resistance ratio. By these criteria, five of the nine patients had a reactive pulmonary vascular bed and responded to prostacyclin administration. In addition, they all responded to nifedipine. The remaining four did not respond to either drug. There was a close correlation (r = 0.85, p less than 0.01) between the magnitude of the pulmonary vasodilator response to treatment with prostacyclin and nifedipine. There was also a significant inverse correlation between the age of the patient at the time of the study and the pulmonary vasodilator response to administration of prostacyclin (r = 0.91, p less than 0.01) and nifedipine (r = 0.82, p less than 0.01); ie, both drugs produced a greater fall in pulmonary arterial pressure in younger patients with primary pulmonary hypertension than in older ones.