A method is presented for an evaluation of the hemodynamic significance of a stenotic lesion in the arterial tree.Twenty-three patients were examined with arteriosclerosis obliterans and intermittent claudication of the same severity. Flow velocity data obtained by angiodensitometry and viscosity values calculated from the hematocrit were inserted into the Poiseuille's flow formula to obtain the pressure drop across a stenotic lesion in the left external iliac artery. By the same way, the pressure gradient was calculated in 33 "normal" subjects.The normal pressure gradient along the external iliac artery varied between 23 to 110 dynes/cm(2) (52 ± 24 dynes/cm(2) for mean and S.d), and the normal resistance to flow was 6.08 ± 4.1 dyne sec/cm(5)).Stenotic lesions of similar dimensions gave widely varying pressure drops (114-4,736 dynes/cm(2)) (mean and S.d 1,309 ± 1,224 dynes/cm(2)) indicating a difference in the hemodynamic significance of the various lesions. These values were significantly different (p(t) < 0.001) from the normal values. The resistance across these stenotic lesions ranged between 21 to 768 dyne sec/cm(5) (196 ± 192 dyne sec/cm(5)) for the mean and S.d and this was significantly different from the normal group; p(t) < 0.001.Direct measurement of blood viscosity coupled with angiocinedensitometry at rest and after forced vasodilatation should provide an accurate means of determining the relative significance of a stenotic lesion and distal vessel disease in a given patient on blood flow to the leg.