The lungs from eight patients who had received thoracic irradiation for the treatment of malignant disease have been studied and a morphometric analysis of the vascular changes present has been made. We found a marked difference between the changes we observed in the pulmonary vessels and those described in systemic vessels after exposure to irradiation. In the former the changes are more pronounced in the venous circulation, and in the latter they are described as being almost confined to the arteries. This difference is a reflection of the difference in oxygen saturation of the blood in the two types of vessels. We suggest that this difference can be explained by the possible role of super-oxide free radicals in the generation of these effects.