The muscular pulmonary arteries were studied by morphometric methods in 25 long-term cigarette smokers and were compared with those of 14 lifelong nonsmokers. Muscular pulmonary arteries < 500 microns in external diameter were identified and counted. The external diameter, medial thickness, and intimal thickness were measured in each of these arteries. Smokers had an increased number of transected muscular arteries < 200 microns in diameter (p < 0.03), increased medial smooth muscle (p < 0.02), and more intimal thickening (p < 0.04). Among smokers, these vascular changes correlated significantly with the severity of small airway disease and with the degree of emphysema, but not with bronchial mucous gland enlargement. We conclude that regular cigarette smoking is associated with morphologic changes in the muscular pulmonary arteries that evolve in parallel with small airway disease and emphysema. Although the functional significance of these findings is unknown, they may be important in the eventual development of pulmonary hypertension and cor pulmonale.