In a retrospective study using histologic material from resected pulmonary ( n=113), renal ( n=26), and bowel ( n=26) carcinomas, the frequency of neoplastic invasion of arteries was investigated. In contrast to 3.8 per cent for both renal and bowel carcinomas, at least 58 per cent and probably as many as 87 per cent of lung cancers invaded pulmonary artery branches. Arterial invasion was followed by thrombosis, organization by granulation tissue, and scarring. No correlation was found between neoplastic invasion of arteries and histologic type and grade of tumors or lymph node status at time of operation. The elastic laminae of arterial walls are apparently not a sufficiently strong barrier against neoplastic growth. The resistance of systemic arteries most likely depends on the higher blood pressure in their lumina, which results in a steep pressure gradient through their walls that successfully opposes the cancer advance. The lower pulmonary artery pressure appears to be insufficient to prevent this tumor cell progress.