The number of smooth muscle cells per unit of surface area of the media of muscular pulmonary arteries was assessed and expressed as an index of medial smooth muscle density. The relative medial thickness of these arteries was also established. Subjects were ten children and ten adults in each of the following conditions: normal, congenital cardial defects with a left-to-right shunt (CCD), primary pulmonary hypertension (PPH), and mitral stenosis (MS); ie, 80 persons in all. The density of medial smooth muscle is generally the same in normal control subjects and in patients with CCD or PPH and is independent of the medial thickness. The index is also the same in children with MS, but significantly reduced in adult patients with MS, apparently by a prominent contribution of collagen and edematous ground substance to the structure of the media. This may explain, at least to some extent, the discrepancy often observed in adult patients with MS, of a very thick media associated with mild elevation of pulmonary arterial pressure. The present study leaves open the possibility that other factors, decisive for this elevation of pressure, are involved. The differences in medial structure may also account for some hemodynamic differences between adults and children with MS.