Modified isotope clearance tests for the objective investigation of intermittent claudication of the lower limb were performed in 14 normal subjects with clinically patent arteries and 9 patients with calf claudication and arteriographically confirmed peripheral vascular occlusion. The tests involved a standard exercise performed in the supine position. During rest periods the limb under test was subjected to simultaneous venous occlusion plethysmography. In the patients with intermittent claudication, results were abnormal when compared with those of the normal group. The modified testing procedure described gave discriminatory results for each group. Their usefulness in monitoring disease progression, treatment benefit, and patient selection for surgery is discussed.