Quantitative structural analysis of peripheral airways and arteries in sudden fatal asthma. The peripheral airways and the adjacent muscular pulmonary arteries were studied by morphometric methods in the autopsy lungs of six asthmatic subjects who died suddenly during an asthma attack, and they were compared with those of six control subjects who died of other causes and had no history of respiratory diseases. Bronchioles of asthmatic subjects had an increased amount of lumen occlusion (p less than 0.01), smooth muscle thickness (p less than 0.001), and inflammatory infiltrate (p less than 0.001), and both mononuclear cells and eosinophils contributed to this increased inflammation. The muscular pulmonary arteries adjacent to occluded and inflamed bronchioles did not have the morphologic features of chronic hypoxia, as shown by the normal medial and intimal thickness, but they had an important inflammatory process in their walls that was particularly marked at sites adjacent to airways. Although the functional significance of these findings is unknown, they may be responsible in part for the gas exchange abnormalities observed in acute severe asthma. 