Nineteen limbs in 15 patients with atherosclerosis obliterans of the femoral artery causing intermittent claudication were studied by serial arteriography. A total of 109 arteriograms were obtained. The shortest span between the first and last arteriogram in an individual patient was five, and the longest nine years. Progression of symptoms was noted in only 2 limbs. There were 2 deaths, 1 from pulmonary disease and 1 from a cerebrovascular or cardiovascular accident. There were no amputations. Progression of atherosclerotic lesions was seen arteriographically in 14 limbs and was absent in 5. Progression was noted in the upper segment (the one subjected to full arterial pressure) in all the 14 limbs that showed progression. Only 2 limbs showed progression in the lower segment also. It was observed that in the majority of cases, the gradual progression of the atherosclerotic process centripetally from the block in the lower extremity was not dangerous to the limb because of the continuing development of collateral circulation. The suggestion is made that the segment below the block is relatively protected from atherosclerotic progression because it is not accessible to mainstream flow and pressure. It is further theorized (though not proved by these data) that when centrifugal progression occurs, it is, by virtue of its greater opportunity to cut off collateral circulation, more likely than centripetal progression to endanger the limb.