To assess the effect of minoxidil on pulmonary hemodynamics, we performed cardiac catheterization on ten patients, prior to the administration of minoxidil, at six months after its addition to their existing drug regimens, and on seven patients following 12 or more months of minoxidil therapy. Systemic blood pressure and systemic vascular resistance were significantly reduced at six months and following 12 or more months of minoxidil therapy. Before receiving minoxidil, nine of ten patients had elevated pulmonary vascular resistance. There were no significant changes in pulmonary vascular resistance following the initiation of minoxidil. The data suggest that pulmonary hypertension is common in patients with long-standing poorly controlled systemic hypertension, and that minoxidil, in doses sufficient to significantly reduce systemic blood pressure, does not cause or aggravate pulmonary hypertension.