Lower leg blood flow was measured at rest and both during and after graduated bicycle exercise in five healthy men and in seventeen patients suffering from various degrees of obliterating arteriosclerosis of the lower limbs. A thermodilution technique was used for flow determinations. The subject exercised in the sitting position and the work load was increased stepwise from a starting load of 100 kpm/min (100 kpm/min load increment every second minute until exhaustion). Three flow phases were depicted during and after the exercise: the aerobic phase, the phase of relative ischaemia and a postexercise phase. During exercise, lower leg blood flow increased approximately twenty times in healthy subjects, while in the arteriosclerotic subjects there was a two-fold to ten-fold increase in flow. In patients with serious distal and proximal stenoses a proximal steal phenomenon was demonstrated during submaximal and maximal exercise. A close correlation was found between maximum individual work load capacity and maximum lower leg blood flow (r = 0.71, P less than 0.001). In the patient group lower leg blood flow at a certain work load was 45% (P less than 0.001) higher in the sitting than in the supine position.